Episiotomy confers no benefits. The most effective way to prevent perineal damage is to avoid episiotomy. Episiotomy, albeit rarely, has resulted in the death of the woman from necrotizing fasciitis.

Episiotomy increases all the bad outcomes it was supposed to prevent. In a 1983 review of episiotomy during the years 1860-1980, this review found that episiotomy has no benefits and causes more 3rd and 4th degree tears, more short and long term fecal incontinence, more bleeding, more pain, and more short and long term sexual discomfort than not cutting an episiotomy. (1)

Most fecal incontinence is a result of damage caused by an episiotomy. (2)  A previous episiotomy is the biggest risk factor for perineal damage on subsequent births - 55% of women who had previously had an episiotomy needed to be sutured at subsequent births. (3)

The World Health Organization recommends episiotomy for the following indications only, but none of them are backed up by evidence:

1. Fetal distress in the second stage of labor, to speed up the delivery of the baby - The truth is that episiotomy is not performed until the head is crowning and about to emerge because before that it would result in life threatening postpartum hemorrhage.  True fetal distress does not develop suddenly in the last 5 minutes as the head is about to emerge. The fetal heart may go down while the mother holds her breath to push.  If the baby has had a reassuring heart beat until crowning, there is plenty of time for the fetus to recover a normal heart beat between contractions, by encouraging the woman to breathe normally.
2. Previous third or fourth degree tear - The opposite is true. Episiotomy increases anal damage. (2) Women with previous severe tear can prevent repeat tears by stretching the scar tissue with EPINO birth trainer before birth.
3. Complicated vaginal delivery, e.g. shoulder dystocia, breech, forceps or vacuum deliveries - No evidence for this. An episiotomy just makes a lot of bleeding so you cannot see what you are doing with shoulder dystocia, breech, forceps and vacuum births.  Delivery of the head is not delayed by the perineum, but by the pelvic bones, so cutting an episiotomy does not speed up the birth and may delay it because of the bloody mess.
4. Maternal stress due to exhaustion or heart failure - Think about it.  The woman carried the baby for 9 months and went thru hours of labor, but suddenly WHO justifies making a deep cut to save 5 minutes?  She will push for an extra 5 minutes if you just tell her that it will prevent 10 days of pain from an episiotomy and a 1% risk of permanent incontinence of feces.
5. A very tight perineum that prevents delivery - Where the fetal heart is reassuring there is no rush.  The perineum will stretch eventually if one waits patiently.

Episiotomy became protocol in the absence of research into what happens without episiotomy. From 1940-1990, in most high-income countries, 100% of women having their first child vaginally in hospital underwent episiotomy.  Only in 2006, was the first study published documenting what happens to primiparous women in the absence of episiotomy.  It found that among women who had vaginal hospital births, had all the usual interventions of vacuum, forceps, Epidurals and Pitocin inductions and augmentation, had an average birth weight of 3500 gm (>8 lbs), but did not have an episiotomy - this study found that 66% of primiparous women had no need for suturing, 33% of primiparas had first or second degree tears sutured and 1% had third-degree tears and 0.7% had fourth-degree tears. (4) 

In a group of low-risk, primiparous women with an average birth weight of 3156 + 334 g and a very motivated practitioner, 94% of the participants had an intact perineum or first degree lacerations not needing repair. (5)

Another report found a 90% rate of perineums not requiring suturing among primiparous women with an average birth weight of 3400 gm.

https://www.researchgate.net/publication/235781466_EPINO_letter_Birth_Journal

Rates of 99% intact perineums among first births in the absence of any episiotomy, with only 1% needing to be sutured and an average birth weight 3150 gm have also been documented.

https://www.researchgate.net/publication/5422360_Episiotomy_hospital_birth_and_cesarean_section_technology_gone_haywire--what_is_the_sutured_tear_rate_at_first_births_supposed_to_be

With a motivated birth attendant, 100% of primiparous women can deliver vaginally by practicing once for 10 minutes with EPINO birth trainer at 38 weeks.

https://www.researchgate.net/publication/235781450_How_to_use_EPINO_for_5_to_10_minutes_and_get_no_tears_on_first_births?ev=prf_pub

Epino birth trainer has received negative feedback, but only by birth practitioners who have NEVER used it.

The modus operandi used to promote episiotomy is: "By doing more damage, we will prevent damage."  This modus operandi has resulted in avoidable maternal death (6) and has been referred to in Lancet as genital mutilation. (7)

References

1. Thacker & Banta. 1983. Benefits and risks of episiotomy: 1860-1980. Obstet Gynecol Surv 38(6): 322–38.
2. Eason. 2002. Anal incontinence after childbirth. CMAJ 166(3): 326–30.
3. Aikens-Murphy. 1998. Perineal Outcomes in a Homebirth Setting. Birth 25(4): 226–34.
4. Albers LL, Sedler KD, Bedrick EJ, Teaf D, Peralta P. Factors related to genital tract trauma in normal spontaneous vaginal births. Birth 2006;33(2):94-100.​
5. Scarabotto LB, Riesco MLG. Use of hyaluronidase to prevent perineal trauma during spontaneous delivery: a pilot study. J Midwifery Womens Health 2008; 53(3):353-361.​
6. Lynch CM, Pinelli DM, Cruse CW, Spellacy WN, Sinnott JT, Shashy RG. Maternal death from postpartum necrotizing fasciitis arising in an episiotomy: a case report. Infect Dis Obstet Gynecol. 1997;5(5):341-4.
7. Wagner M. Episiotomy: a form of genital mutilation. Lancet 1999.5;353(9168):1977-8.