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
That is the main reason I too believe but then they can simply print the private key and they can rest assured that they can't loose their bitcoins.
I have been using two wallets since a long time, but that is actually a exchange, c-cex, they pay a good fee on withdrawals while they only charge 0.0002 so I use them for quick payments and for accepting money from clients I use xapo and blockchain, xapo is sometimes dangerous as they ask ID proofs.