My understanding was that they were only supplied when the particular type of slide you have might be prone to opening. Herk's post have covered this pretty well, but let me throw in an extra example.
My 2012 Flagstaff came with slide locks. My Brother In Law's 2011 did not. While we were trying to figure out the difference, I noticed a few things:
1) My slide rails are inside my TT, under the couch and under the oven. In the BIL's, they're under the floor since his is one of those flush slides.
2) When retracting mine, you can hear the clutch ratcheting. You cannot with the BIL's.
3) This is probably the most important difference. On my TT Forest River included a "Place Slide Lock here" sticker that indicates where the wall was supported to take the pressure of a slide lock. His had no such sticker and a random check with a stud meter was inconclusive to whether or not he had any backing in there. Our guess is that even if he did get a set of slide locks from somewhere, he'd end up damaging his trailer trying to use them because the walls weren't supported for them in the first place.
There's no use crying over spilled milk, unless it's on your keyboard.