Like any "wallet", you can always receive coins to an address without the wallet being open and/or connected to the internet... Otherwise you'd never be able to find paper wallets!
For "withdrawing" (sending coins from an address)... The answer is "it depends"...
If no one else has the seed and/or private keys... Then the hardware wallet would need to be connected to a computer for you to sign the sending transaction with the appropriate private key...
However, if someone had access to the 24 word seed and/or private keys, they could simply import that into another wallet and send transactions...
The idea behind the hardware wallet is that it makes it VERY difficult for someone to get the private keys and/or seed phrase (assuming proper procedures are followed during setup and use)