These are magical in some cases, where other things make no difference or have little effect. For instance, on plastic it can sometimes remove marks that seem ground in (but avoid super-highgloss plastic, or you'll get fine swirl marks in it). You can remove soap scum with just water; genuine ones have some detergent, but even immitation ones need only water and a little more effort.
They are very-very, VERY easily cut or torn, so use the gentlest touch and avoid any rough surfaces. Edges that aren't sharp enough to cut you can still slice right through these.
I often cut them into smaller pieces, especially for things that might darken them (carbon or grease) or wear holes in them. For dishes, a I use a whole one. If you rinse well and gently squeeze away most of the water, they don't usually get that horrible "sponge smell" that you can't usually wash out of sponges. Try not to flatten, so they last longer.
(( Soaking a sponge in distilled water overnight, or in the semi-distilled dehumidifer water, seems effective at killing the bacterial smell. ))