So then... what can a Knight do? I dont wanna feel useless like how most of my friends said Knights are

How can a Knight be useless? Karkat is good at bringing his team together, even if he’s not a fighter, he’s good getting others together. And even if he wasn’t and Karkat was a big failure of a Kngiht, which he’s not, you have Dave in canon as an example of what a Knight can do. Spending all of his Time to aid his friends and those he cares about, using Time Travel and Time Loops even while he risks seeing himself die just so he can ensure the Session goes correctly and that none of his friends die. For them, their Aspect is not a Weapon necessarily, but it is a Tool, and they’re an adept at both Serving others in their needs with said tool, and using it to suit their own needs when they need to do something.

I can honestly not fathom why anyone would think a Knight is useless.