Behind their apparent usefulness, these are nothing else than for-profit data collection companies (a Facebook for academics), aiming at selling ads and related monetising mechanisms (see it for yourself, here and here). They want to get all your contact list too, to spam them about their services.
There are free alternatives (free as in freedom of speech). For a start, Google Scholar (top-left of page here). Then, self-updating repositories for multiple disciplines (economics here). Consider also ORCID (which also supports peer-reviewing certification!). Finally, your own website (trivial to make nowadays, with Google Sites, Github, HUGO, etc.), plus social media.