If you want to win over investors, focus your pitch on how large of an opportunity your company has based on the market demand for your product or services. Investors can only really make money on a startup deal if there is growth potential. And that growth, many times, is dictated by the market opportunities.
If you genuinely feel like your product or service can change the world, then let that passion come across in your pitch. If you simply identified a great business opportunity to make money, that's OK, too. There is no wrong answer in why you started your company -- unless that answer isn't genuine and authentic.