Because that's the fastest way to the front lines! Duh!

And Agent 3 has speed. Inkbrushes are the fastest weapon an inkling can use to move across the battlefield. The monster is blinded, which gives her a chance to dive back into the ink swim under it to the back. There, she slams the Jinnjar Inkbrush onto the thing's tail. It howls in pain, thrashing every which way and destroying the scenery around it.

Oops. That might not have been the best idea.