You don't have to, but it's kinda dumb not to. You can cram it all into a wooden chest and bury it in your backyard if you want, but while its down there its slowly losing value.
Instead you can set it up to make more money for you, without you having to do anything. Money, in sufficient quantities, can become self-replicating unless you do dumb shit with it and waste it. You just hire someone who knows how to make that happen, some financial manager of some sort, and they take care of it. You can even set it up so they just budget out an allowance of your own money for you, so you know how much you can spend every month or whatever. If you want.
This way you don't have to worry about anything, you just know you have such-and-such amount every month to play with, just like most people. It's just a lot more than most people get.