you don't NEED a muffler on, but how long do you want to continue flying? unless you have your own 40+ acres, you will have neighbors complaining and shutting you down fairly quickly. flying without one is generally highly discouraged for this reason!

there generally isn't anything inside a glow muffler, just an empty can. some have a baffle near the back for a little better muffler pressure, but that is removable without much difference in most engines anyway.