This is not a new custom, rather an extension of the responsibility everyone has to encourage others to increase Torah and mitzvos among friends, in a happy and friendly way.
If for some reason this was not so common until now, it is needed now, and as we have seen, it in based on the practices of the great Jewish leaders.
In the darkness of the pre-Messianic diaspora - we need more good deeds that will radiate light.
And this activity is tried, tested and true; it will increase good deeds, Torah and mitzvos.
Since the golus is seen as the time of incubation and the salvation as the time of birth, may our good actions on birth days speedily bring the ultimate birth of the Jewish people - the true redemption.