One way or the other, they are "dimensional adhesives" meant to give elements a little "pop" off the page.
Why bother popping? Well, it can make an otherwise flat layout more eye-catching. The extra layers and dimension are just more interesting. If you want to make a photo or embellishment stand out (pun intended) then pop it up! In addition to creating a focal point, raised elements can give the added impresson of movement. This is great if the subject of your layout contains action.
Some scrappers don't like lumpy embellishments because they're hard and can dent the facing page when the album is closed. Foams squares don't do that. They're soft and squishy so they will flatten before they can do any damage. Even if you're not a 'lumpy-bumpy' scrapbooker, foam squares will add visual interest without extra weight or bulk.