Huh. Interesting question. My RV didn't come with them. Not sure it ever had them. I have several friends with the same or similar RVs and none of them have them either. Or at least years ago when I asked they didn't know what they were.
I don't think they'd be necessary except in a catastrophic failure of my HWH hydraulic system and if I couldn't drive unless the slides were wedged in place, what I'd do is head to the lumber yard and get some sticks of 1"X2" lumber. I'd place those on TOP of the slide and tape them in place. That would prevent the slides from slipping out. If the bottoms tried to pooch out while driving, then I'd add some lower down. Since I'd be well aware of the failure, it's not likely that I'd try to extend the slides with the wood braces there.
But for every day travel? I don't use them and don't know anyone who does. I'm interested in hearing from others that have them and use them all the time.