Trezor is an offline device, hardware wallets are offline devices, however, you can also set up your wallet in an airgapped device, this will give you the same security as a hardware wallet, if you know what you are doing.
It is needed to sign your tx's offline, because your keys should be permanently offline and never connected to the internet, that way it isn't vulnerable to online attacks. Take note that you can store a small amount of BTC in an online wallet, but the large part of your BTC should be stored offline.