Another "history lesson" (Geesh I'm starting to feel old)...
Back when "tip outs" (not slide outs) first became popular in travel trailers, they were "cantilevered boxes" that "flipped into the trailer" for travel and "flipped out of the trailer" for extra space when camping. The design of those "tip outs" necessitated "supports" to keep them level during use.
When the transition to slides occurred in the late 70's/early 80's, initially they also required supports under them (and "travel locks" to keep them from deploying during travel). Through the years, slide engineering has improved significantly and today's slides do not require any "underpinning support".
Not to be "outdone", the manufacturers of "tip out supports" apparently decided to rename them "slide out supports" and continue selling them as an optional item to "improve the camping experience". Hmmmmm
This "new marketing of an old product" keeps them in the "selling game" (making money from RV'ers who read the advertisement and see a "must have"). They usually don't live up to their "hype" and as noted below, can cause more damage than good.....
Now, with today's slide mechanisms, the entire slide is supported by the mechanism and once deployed, becomes an integral part of the RV. The rams (or cable system and slide framework) support the weight of the slide and any movement of the slide frame "must mirror" movement of the RV frame. If the two move independently, the rams (or cable system) can be damaged, causing problems with alignment and operation of the slide mechanism.
Placing "slide out supports" under the slide will cause it not to "move with the rams and the RV frame", possibly bending the rams. This could damage your slide mechanism causing expensive repairs to be needed.
Even though the "RV marketers" claim that slide supports help with stability, the potential damage they can cause (if the RV moves and the slide doesn't) makes them more of a potential problem than an advantage in stability.
Heaven help the RV owner who has slide stabilizers under his RV and has the "highly unlikely" problem of two flat tires on that side of his RV. I wouldn't want to even consider the damage that could occur.....