Is Airtag useful in daily life?

It is useful for items you can lose or misplace.

For example, I have an AirTag attached to my keys.

Have a look at this article:

Add an AirTag in Find My on iPhone - Apple Support

unless you leave or misplace the item you tied it to, then it will serve no purpose for you in your daily life,

sort of like a lock on your door is pointless until the day someone attempt to enter it that you do not really wish to enter without at least knocking.