The same way restaurants and bars give free nuts, chips, popcorn, breadsticks etc.

It's a way to keep you coming back. And each time you use their products they have a chance to upsell you, show you ads, you might like the product so you subscribe to get better features.

That's also why they work so hard to make all their products work well together. So even if you use one product, it's very easy when you need something to use another of their products without going to a competitor (eg opening an email attachment in google docs not in ms word)