these are called profile links / foundation links and they're the easiest (and free) to get, but they're pretty useless in terms of ranking power (at least I've not heard anyone ranking with profile links alone), although the logic behind them is that they come from domains with high authority and in theory they should boost your authority, therefore they should boost your rankings. But theory and practice are different things when it comes to these links, I don't know why (probably because the page where your profile link is placed on these "high authority" websites is actually 0, and sometimes not even indexable...)
You should get some of them for diversity and anchor dilution purposes, and then move on to more powerful links...
