Look, if we don't charge you it's free, right? Of course, we need to pay for it somehow!
Suppose we built a web space hosting service. We would have server computers, communications hardware, lots of telephone lines, one or more people to maintain the hardware, maybe someone to answer technical queries .... a pretty large investment as well as something with considerable on-going costs.
Now suppose that you wanted to use some of our web server space, and some of our communications hardware, and some of our people to keep your web space operating smoothly? And then suppose someone else wanted to do the same thing? Sure adding more disk memory isn't expensive - but that's only a small part of what it would cost us to give you free server space.
So how could we give you no-cost web space? Simple - we would sell advertising on our web space and hope people would click on pop-ups or banners or all that stuff, and enough of that would happen that the advertisers would pay us to run the *free* web space.
And that's the story of *free* web space. It's free to you because someone else pays, and they expect to get something back. And that something is customers who will spend money :)