Ask the flower int he field : 'Do you feel useful? After all, you do nothing but produce the same flowers over and over?'
And the flower will answer : 'I am beautiful, and beauty is my reason for living.'
Ask the river : 'Do you feel useful, given that all you do is to keep flowing in the same direction?'
And the river will answer : 'I am not trying to be useful, I'm trying to be a river.'
Nothing in this world is useless in the eyes of God. Not a leaf from a tree falls, not a hair from your head, not even an insect dies because it was of no use. Everything has a reason to exist.
Even you, the person asking the question. 'I'm useless is the answer you give yourself.
Soon that answer will poison you and you will die while still alive, even though you still walk, eat, sleep and to have a little fun whenever possible.
Don't try to be useful. try to be yourself: that is enough, and that makes all the difference.