This is for the sake of safekeeping. Having a single wallet is prone to some uncertain events such as hacking and forgetting the private key or passphrase. So in order to lessen the casualties, some user distribute their coins on different wallets. This way, if a wallet was subjected to a disappointing event, then the user will lose only a portion of his coins and not the entire coins in possession. Some also use different wallets for different type of transaction so that confusion will not take place.