Because it provides independence and mental freedom, what I try to say with this, that in the measure that one develops an activity additional to its main activity in such a way that it can be carried out without harming the execution of the other that it does on a daily basis, that secondary activity is will consolidate with time, until reaching a point that can become a main activity or not, that will always be a choice of who carries it out.
But what should never stop contributing this secondary activity, is an air of freedom, because the worst thing that can happen to a person is not having options and feeling imprisoned in a place, just because they do not know what to do.
When that happens and that fear of losing your current job, makes you not enjoy your work because this fear that you should stay there beyond all because you have no choice and the design of Plan B is about generating options , hence its importance and value.
On the website that I maintain with my family we teach you how to design this plan B.