Decentralization.
What do you mean by that? Like we don't have to depend on a back-end server like we do a with Trezor?
Let's say you have a three company owners. One is in Japan, Second in the US and third in Zurich. You may need two of them to sign company expenses. That's what multi sig is about.
Second use case: You have to sign transaction with 3 out of five: your Trezor, your wife's Trezor, your mobile phone, Trezor in the bank vault and your geek cousin computer wallet.