Let’s take a (not so) hypothetical situation: assume you hold some kind of responsibility in your group. You might the the principal investigator, a researcher or even just an experienced post-doc; the important part is that you are somehow responsible (morally if not practically) for people hierarchically below you.

Let’s also assume that a new PhD student (or, as a limiting case, a fresh post-doc) enter your group. The path he/she took to reach your group can vary enormously from country to country so let’s skip it. The main point is that you don’t really know this person but you have some good reason to believe he/she will do good.

The first few months are there to allow your new PhD student to get used to the new place, the new subject, the new “way of doing things” and so on. After some time passed you expect him/her to become productive. After all he/she is no more a undergrad student and is reasonable that, given some guidance, he/she will start conducting an experiment (or at least a part of it). After all the very reason you took in a new PhD student is because you need people working.