In reply to the original post: no, I don't always buy minis for a specific use or project. On the contrary.
Over the years I bought a lot of minis without even knowing what to use them for, simply because I liked them. I enjoy painting for painting's sake. Sometimes figures have been finished for years before a ruleset arrives I can use them with.
Sometimes you could call it a project, like Vietnam of Prehistorics, but at the time I start painting I have no idea for what rules I am painting them. That usually comes along in time. My experience is that it is more fun to buy and paint minis that you enjoy painting and then go looking for a ruleset that you enjoy playing than picking a ruleset and start sloughing away at painting an army you might not enjoy painting.
In my absolutely not humble opinion the only waste of time in hobby activity is when you are doing something you do not enjoy.
STOP THIS IMMEDIATELY AND MOVE ON!!!!
But seriously, when you catch yourself not wanting to paint specific figures, go and paint something you do enjoy.