Scatter symbols have no need to be a part of a winning combination. They can be "scattered" around the reels as long as they're on the screen.
Scatters are beneficial in different ways depending on the slot machine:
In slot machines with bonus features, scatters usually initiate the bonus game. One scatter may be enough, in which case additional scatters make the bonus game even more beneficial, or the bonus game may require three scatters to begin at all.
Scatters may also be used as win multipliers; for example, one scatter multiplies your win by three, two catters multiply your win by six, and so on. However, in case you have no winning paylines, there are no winnings to multiply.
And they may simply give you significant winnings or free spins (in which case the amount of scatters usually dictates how many free spins you get).
Scatter symbols can't be replaced by wild symbols - in fact, scatters are the only symbols that wild symbols are unable to replace.
Slots with scatters are known as "Scatter Pay Slots".