I am an adult who needs bottom braces. what are the consequences of not getting them?

Depends: Firstly, orthodontics is an elective procedure. No one needs braces. If your teeth are very crowded and you have trouble cleaning them effectively, then the consequences could be periodontal disease, and there is an association between oral bacteria and heart pathology. Do not be "scared" into treatment however. If you are a diligent brusher/flosser this need not be an issue. Go for it!

Maybe no problem: Certainly there is not anything necessarily terrible that will happen if you do not have the lower teeth straightened. It depends on what is the problem i.e. Are you unable to keep the teeth clean because of crowding? Then there is a need. If the lower anterior teeth are in traumatic occlusion there is another justification. See an orthodontist and get an opinion.

Your smile: While it may effect your bite and overall dental health, most likely the biggest issue will be that you will not be happy with your smile the rest of your life.