company law question

Why is Abridged , Shelf and Deemed Prospectus made? Why not just issue a normal Prospetus?

Abridged prospectus.

In case of Abridged prospectus,i think it is less work and less time consuming,less expenses than making a Prospectus.m i right?

(as it is the shorter verson of the prospectus containing all the sailent features prospectus)

But what are other two needed for?

thank you.