For a side project? I build it if I want to use it. There is a totally different goal for a business, but side projects are hobbies. They bring value to my own life regardless of what other people think of them. It is a great bonus if other people also get value of it and are willing to pay something, but there is nothing wrong with writing a project that simply helps me in my own life.
You're right. I meant a side project that you build for profit.
I totally didn't imply it's bad to build something for your own use without making profits of it :)
How would you formulate this question?
In that case, I wouldn't call it a side project at all. Whether it is your full-time job, or something you are building as a part-time business, the validation of the demand doesn't change -- Make a prototype and ask people if they'd pay for it. If they say yes, move on to an MVP. If not, ask for feedback and either try an updated prototype or try something else.
