Some elegant answers but this part nails it:
1. People’s lives are improved by fun apps and games, in the same way they are by movies, music, food, and books. We don’t usually ask why people waste time in those fields. Kirsten Dunst is never asked why she makes movies instead of working on gene therapy. Nor do we suggest that we stop making movies because we already have plenty of those.
2. And, what starts out as trivial often ends up being the most impactful of all, as pointed out in one of the comments. Examples:
- “Trade Pez dispensers online” -> a marketplace larger than many national economies (eBay)
- “Tell your friends what you are doing” -> a global communications network (Twitter)
- “Hook up at Harvard” -> a billion person network, disrupting governments (Facebook)
- “Send money to your friends” -> disruptive payment network (Paypal)