It's difficult to say that's what they're doing.
Google offers its storage space in return for serving you ads when you use their services. And they also have access to all your data, which makes it possible for them to learn about your lifestyle and, in turn, again serve you (better targeted) ads. So they probably make some money on it. They also give you opportunity to purchase additional quantities of capacity.
Microsoft, on the other hand, bundles their One Drive with Windows purchase. So you could say that you bought the capacity when you purchased your Windows license. You could also say that this is one way for MS to keep you tied to their products, which will make you purchase another license in a few years. And, again, you can buy extra space if and when you need it.
Dropbox is a standalone product. They have in the past (I don't see those offers anymore) partnered with phone vendors, to lure you in. The idea was that the convenience of having 20 or 30 GB of space (or whatever amount they "gave" you), would force you in paying them, after the honeymoon was over.
So far I haven't met a single person who did this. So they are probably doing something a bit wrong here.