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D&D 5th edition rules - usage of spell attack modifier

We are newbies trying to get the rules down, and while we have mostly been able to resolve our own questions we still have some we haven't been able to answer.

Player handbook on the Wizard Class page 114 defines a Spell Attack modifier , but the spells I have looked at all specify attack damage without using this modifier or mentioning it. I can't determine what it has to do with anything. Where does it come in to the game?