Genjutsu can be useful in multiple ways in battle. It can be used offensively and defensively. On offence, genjutsu can cause mental/emotional damage to ones brain. You can trap someone in a genjutsu and mess with their brain to the point where you are in control of the battle and even kill them. It can also be use to defend, you can use it to throw of your opponent, tricking them and using it to counter. For example, Itachi uses genjutsu perfectly in both situations. He can cause serious damage like what he did to kid sasuke with Tsyukinomi and do the other like the time against Naruto in part 2 where naruto was trapped in the genjutsu and could get out. Itachi throw him of badly.