Funny, I was thinking about this and was about to pose this question until I saw it had already been posted. I think necessity has created the drive for the majority of man's inventions, but I leave room for tinkering, curiosity, and even accident as alternate "mothers of inventions". Inventions which are developed in response to defined needs probably have a higher probability of success and widespread use. In the modern world investment plays a key role and investors are more likely to fund product development when the invention addresses a necessity. We still see the occasional surprise invention that didn't result from a concerted effort to meet a need.