Great question! In some cases I've seen signup's increase on a site up to 800% almost over night because of model boxes like in OP's images, but it is guaranteed to increase user bases on any site that they are used.

Businesses most likely will not profit directly from a user inputting their information, but what the company does with that information is where the huge amounts of cash come in. You can sell the information to 3rd parties and get (depending on how much information comes with each user) no less than $0.15 per user, I've seen information sold for as much as $3 per user.

Another way to make money from this is through email blasts advertising for other companies, products you sell or whatever you want.

To be on the internet and serve the community as a whole is very expensive and someone has to pay for it, nobody is going to put a website online and not want to profit from it, especially if there are hundreds of thousands of views on a site.