There are many reasons to use a microphone. The obvious one is so that people can hear you, or hear you better. In a similar vein, using a mic reduces wear and tear on your voice ― always a good thing. Or if your presentation is being recorded, using a mic will greatly improve the audio portion of the recording. So the question really isn't "should I use a microphone?", but rather "is there any reason why I shouldn't use a mic?" If you don't have a good reason for not using a mic, it's best to use it.