Why would I provide a composer.json for my modules?
I wrote a blog that gives some case studies about when and why you would include a composer.json with your module: https://www.morpht.com/blog/drupal-and-composer-part-4-composer-drupal-developers

Basically it comes down to four things:


It allows your module to be managed with composer.
It allows your module to have it's Drupal module dependencies managed by composer
It allows your module to have it's 3rd party library dependencies managed through composer
Composer handles autoloading of namespaced classes in libraries it manages, meaning your module can use the 3rd party libraries it depends upon, without having to include any files


If none of the above requirements match your needs, then there is no reason to have a composer.json file in your module.