The very first reason is probably because people simply do not want to keep all their savings in one purse. If suddenly one of their wallet is broken, then the remaining coins remain intact on other accounts
yes that is very right. to me in fact most of the people are using online wallet, which may not be so much secure, therefore when people have more bitcoin in their wallet then there is more risk of losing their money, therefore most of the people distribute their money in different wallet, where they feel a little safe. to me I am using different wallet for different purposes. for signature campaign and for spending money I have separate wallet which mostly use for spending btcoin, while for trading I have another wallet, which I only use for trading and holding bitcoin for investment purposes.