posting your auth first started on twitter, so dapps could link addresses between twitter accounts. farcaster already has that built in, you can use normal wallet auth to authenticate wallet ownership and then use FC to find the corresponding FC account. posting auth here is just forced marketing
wallet auth can be annoying too (someone using hw wallet) we wanted to do auth via casting because we thought it would be simpler (marketing benefit as well), but it breaks often b/c it relies on Farcaster notification API which can be spotty waiting for a better solution