Android devices are easily hackable but that's besides the point.

Giving your Nsec is unrestricted access to your account, whereas with a signer you know what's going on with your account so if weird posts start popping up after using a new app you can just revoke access.

It's like having your main home, and your garage.

If you give an NSEC, you're basically giving the keys to your home to everyone, whereas with a signer it's like giving a key to your garage that connects to your home.

You can let people into your home but not give them full unrestricted access to your home, they still have to go by you for full access.