There are quite a few reasons why you might want to do this. The stable money from your day job can help you still be able to eat, pay rent, and all of those other wonderful things while developing your business on the side.
There is the obvious downside of not having as much time to concentrate on your business, but there are some other gotchas that you might not be aware of/have forgotten about.
- Employment Contract
- Non-Compete Agreement
Most employment contracts are very clear, any work you do at work is the property of your work. You have to be very diligent about separating your business from your day job. Another gotcha you should be careful with is using company equipment, say if you have a company laptop you probably should restrict using that strictly for company use.
A lot of tech firms have a non-compete agreement. Now the enforceability of the non-compete agreement is a completely different subject, and you may wish to talk to a lawyer about that. For the most part, most non-compete agreements at the very least prevent you from entering the same line of business as your present firm for a fixed period of time. So if the firm you are at right now, makes file transfer software, and you have a genius idea about how to make better file transfer software, your probably out of luck.