The forms in intellect are some kind of blueprint or model on which the physical world is based, and something needs to come along and shape the matter in accordance with that blueprint. What does that is soul.

The importance of Plotinus is not only his own philosophical ideas which are quite interesting, but his historical impact as the founder of neo-Platonism which is the main philosophical tradition of late antiquity.

In fact Plotinus thought not only that soul in general is eternal so that you always have soul, but he thought that each person's soul is eternal.

I'm sure that Plotinus wasn't that impressed by the Christians.

Soul puts the determination or forms or images of forms, into matter.

Just the way you might look at a painting and see the painting, and the painting is outside you, so this immaterial intellect would see the forms and behold them, as if they were standing before it. And Plotinus said that that can't be right because it falls prey to sceptical objections.

Plotinus is usually called the Founder of Neo Platonism and what that means is that in this philosophical circle, he was founding a kind of renewed attempt to understand the thought of Plato which however they were combining with the thought of other philosophers, including Aristotle, the Stoics and Pythagoreans and so on.