Most of them buy altcoins too with bitcoin investment.Mostly, people get into altcoins as a way to hedge against Bitcoin. Here are some common reasons for someone to hedge with altcoins:
1,Bitcoin may have some catastrophic failing that the altcoin is not subject to.
2,An altcoin may have some future utility that is significantly better than Bitcoin allowing the altcoin to overtake Bitcoin.
3,Even if Bitcoin continues to be the most valuable cryptocurrency, there may be room for altcoins to serve some other niche and hence             may not be bad bets in their own right.