I do so as well, this may affect your growth opportunities, but you could look in the future without everyday fear running out of money and not being able to pay rent or kindergarten.
Seems to make sense that while attempting to start a venture knowing there's a significant risk of failure, one might want to have a separate stable income stream.
Also, the regularity of a current gig can be motivating... instead of having "infinite time" one must use their days more efficiently to get a fresh endeavor off the ground, and in my experience this cultivates a healthy industriousness, if you're into that word.
So true. Really weird, but really true.