Black shows scratches much more then other colors so clear coating will help hide the surface scratches and give some more durability to the finish. some coaters will always try and upsell the clearcoat because they make more money doing it. The gloss black I use isn't any less gloss than the clear coat so it is not a necessary step for anything other than the above mentioned issues. If you have a satin black you will need a satin clear. If you want gloss black it doesn't matter what percentage of gloss black you use, if you use a gloss clear on top of it, it will be gloss black.
You can sand your base coat for better adhesion of the clear coat. It will turn the black gray but as soon as you apply the clear it will bring the darker black back.