These conventions are rarely very strict but they do help communicate your ideas. Understanding them and understanding the impact of not using them is an important set of knowledge if you want to be clear and well understood in your writing.

It is similar to good typography in that it isn't required but that it will often make the reader enjoy or understand the work more when reading.

Obviously there have been many great writers who knew how and why they were breaking those conventions. E. E. Cummings was a notable example.