Breed Sow speeds up the process I think.
It doesn't. At least, not any more.
It reduces the pregnancy penalty by one level. No penalty for showing, light penalty (-20 phys) when she's about to pop.
It's useful if you're planning on breeding phys reliant slaves/peons, since it means they aren't down by 40 phys for the better part of a month. Also useful for transferring stat potential between races.
Let's say you invest a lot in a starter slave, and get their stat factors way up through a combination of straight upgrades and classes that increase them- but the race of this slave isn't ideal for whatever role you want to breed for (as an example, Dark elves and Fairies are ideal for spellcasters, although fairies are a bit too min-maxed to really be worth it IMO.)
You can give your starter slave breeding sow and breed them until RNG gives you the race you want with the mothers factors. (race is variable- but factors are always the mothers.)
TBH though, from a pure utility perspective, breeding sow is pretty overpriced for what it does. You don't really need that kind of stat optimizing to do much of anything (at least, in the current game)- and while the penalty reduction is nice, you probably aren't reliant on the productivity of a slave you earmarked for breeding anyway.