what is a sewing pattern and why is it necessary?

I dont understand why you cant just purchase fabric, get the “model” dummy to drape the clothing on, pin and sew, whats the point of a pattern?
if you do need it, how do you get the shape/ cut you want for the look?
thanks

The technique you describe is called ‘draping’ and is used by many dress makers and couturiers. It’s a lot more difficult to get right than using a ready made pattern, unless you have been taught how to do it properly.

A dress stand, even one made to the exact measurements of the person for whom the garment is to be made, is no substitute for fitting properly, and you need to know quite a bit about garment construction and fabric characteristics in order to get it right.

Flat pattern drafting starts with a block drawn to a set of measurements (either a standard set or those of a particular person). Style comes from manipulating the block pattern to add details. This is not particularly difficult to do, once you know the principles, but needs a good eye, a lot of patience, and a facility with both drafting tools and numbers.

How to Sew : Basic Sewing Patterns