pre-school is the new kindergarten.
In fact in many areas they now call it K4 instead of pre-school, so that would mean "pre-school" would start at 3.
Kindergarten used to be where kids learned how to behave in a group for a set amount of time, how to socialize, how to follow complex directions and how to begin to control impulses. Not anymore. If you don't know these things already IN ADDITION to the basics of sight words, simple math/counting and personal care a kid can really struggle.
My daughter's first week of kindergarten (5 years ago, she had just turned 5) they were given a list of books to read each week and a worksheet book report that was turned in each Friday. They had 2 or 3 "assignments" each night in various subjects and the ENTIRE school day was spent in actual school, going from subject to subject that even included introductory spanish. And this was just a public school.
There are a good many moms who have the time, ability, experience, stamina or whatever to teach them all the pre-stuff basics at home. There are a good many moms who, for whatever reason, can't. So, pre-school teaches the "what is school about" to your child before they are held accountable for behaving in school.
I also think, it's extremely valid for a mom to need a "break". You sorta said that like it was a bad thing. I was both a SAHM and a work-outside-the-home mom with one kid who is pretty well behaved. In order to really focus on your kid there are times when you need somewhere for the kid to go because you have to focus on something else at that time. I didn't want to be a mom who was doing laundry and cleaning while I parked my kid in front of the TV or whatever, so even when I was a SAHM my daughter went to a "pre-school" 3 days each week in the morning. That's when I did shopping, major cleaning, scheduled my appointments, sat and stared at the wall to keep my sanity whatever.
I think that experience was great for her, she built some really great friends and is very independent. We had NO issues with her crying when I left like we saw in kindergarten because she had a basis of expectation of what would be going on and since her experience was good it was something she looked forward to.