You trade your present time for future return while working on side projects. You might have different outcomes, your side project would have got traction, you can move on with idea to startup. Or it may help you to find early invester or co-founder through that.

If everything else failed, side-project definitely helps to find a suitable technical job that fits your resume, because you spend a lot of hours on working on technology you liked. When you are not succeeding, don't forget to open source your work. That add credit, too.