Step 1: When the perineum is incised to widen the birth outlet, the surgeon can angle the cut either straight down (midline) or off to the side (mediolateral).
Step 2: By angling the incision laterally, the line of cutting moves away from the anal opening. This protects the anal sphincter and anal canal from being torn during a difficult delivery, which is the main benefit.
Step 3: The trade-off is that the mediolateral cut bleeds a bit more and is somewhat trickier to stitch, so options about less blood loss and easy suturing actually favor the midline approach instead.
Step 4: Hence the correct rationale for choosing mediolateral is sparing the anal sphincter and canal from injury.
\[\boxed{\text{Reduces damage to anal sphincter and anal canal}}\]