Reason: seedsigner holds no data, it's always ephemeral. Your keys are reentered whenever it's time to sign. Seedsigner prevents you from relying on electronics, like other hardware wallets.

If your "main" backup is in an electronic device, like the secure element of a cold-card, you are underinformed about reliability of electronics. Cold-card is a runner-up to Seedsigner, but it's single-source and not fully opens source hardware and some specific components. It's more convenient, and good to hand to the less technical if you are asking your brother or lawyer to be your backup in a multisig. But, in any case, you still need to backup your seeds in a more robust way than in an electronic wallet.

Seedsigner is only an offline signing tool, not a wallet. It is open source, can be replaced, and it's not necessary to move your bitcoin.

Your main backup should be in stainless steel. You also need the wallet derivation path (often overlooked) and for multisig, you also NEED the xpub of the absent wallets. Usually this is in the form of a wallet descriptor QR.