You also plucked your private key (nsec) into whatever client/app you used to post this.  Now they have it.... and the next app/client... and the next...

Many of the apps/clients are new, experimental and buggy.

They could get hacked or sell or leak your private key/nsec... the password to your identity in the nostr world.

This is what Alby or another 'key signer' does.  They provide proof that you own your public identity without handing over your private key.

Seems trivial at first, but it's important.