to put it into perspective, with a simple genjutsu, a caster can visually show memories or thoughts. madara made himself young in an ilussion and back to old again, or can even look into the receiver's mind.
in sasuke vs danzo, sasuke used a very basic illussion to make it seem for danzo that one of his eyes on his arm are still open, despite it being closed actually.
or it can spawn some illusionary people, without changing the background.
or use it offensively, like make an illusion of a giant holding the receiver in place, and so he is immobilized.
or genjutsu to knock a person out or to put him to sleep.
it can be made so the receiver would perceive reality differently (like things in different places than they look, or the number), for example:

Itachi vs Deidara Full Fight (English dub)
also people can be controlled via genjutsu.
those are only regular genjutsu.
if we take away IT, then naruto team can still win in a regular way, or with the help of sealing.