What do you have a multisignature wallet if you are the only one that is going to manage that wallet? why should I want 3 or more private kays instead of one?
Maybe you have a 2of2 wallet, with one set of private keys stored on a permanently offline machine (in my case, I've used a laptop with its wifi card physically removed). That way you have the security of a cold wallet, but with most of the convenience of a hot wallet. You could just as easily ask "Why buy a Trezor when I can manage my own keys offline?" and the answer is the same, it's a way to combine security and convenience. I could pick up a used laptop for less than a Trezor would cost me, wipe it and install Ubuntu in about an hour, pull the wifi card and I'm good to go.. but none of that is as easy as plugging in a USB device and pushing a button when I went to spend.