They stabilise the switch tops, which helps to stabilise the rails and reduces wobble just a little. Tightens up the whole switch assembly. They also look cool.
Most people don't think they're worth using, but I do think they're worth it IF you want the most stable and best feeling MX board. Of course this means doing a bunch of other mods, too. So I don't think it's worth ONLY adding stickers, it's a lot of labour for little reward.
On my boards I change the springs (for ErgoClears), add stickers, trampoline and latex mod the switches all at once. Adding the stickers takes the most time of all the mods, though, since they're quite fiddly and you have to place them accurately.
