Why do I have to change Dictionary Form into Polite Form?
It is highly recommended that you do that when you speak or write in Japanese because you can be polite to everybody in that way.
People would understand you if you use a verb (or adjective) Dictionary Form, but it does not sound polite. You should use Dictionary Form, only when you speak or write to your friends or close people.
Being polite does not cause any problems. Nobody hates you to be polite. Learn how to use Polite Form properly if you are a beginner, then start to use Dictionary Form gradually, depending on a situation.
たべる and たべます both mean ‘eat’, but one is polite and the other one is not.
たべる（Dictionary Form）→ does not sound polite
たべます（Polite Form）→ sounds polite