Being everyone’s free storage unit does not generally make money.
So they have to turn the resume storage part into a loss leader and turn a profit some other way. Being a place to list job postings is the most obvious but they have competition already.
Throwing on ads will make money but LinkedIn isn’t a site most people stick around on. Turning LinkedIn into a social media site keeps people on the site so the ads work, but it’s a pretty ugly way to do it. The type of people who are actually going to use LinkedIn like Facebook is probably the worst kind of people you want sticking around too.
LinkedIn could have stayed a small site with a small team but they got started by getting money from investors. Now they have other people to answer too.