To communicate effectively, that's why.

It could be as simple as that.

Strong, clear writing is a microscope focused on your brain.

Strong, clear writing indicates you have clarity of thought, that you can organize, can think logically, can express yourself well.

An inability to express yourself clearly via the written word could be interpreted as an inability to carry out tasks, to comprehend, to understand, to grasp, to create, to be original. Some of your readers might interpret poor writing skills as a clue to a low level of intelligence. Is that what you want?

Of course not.