A microphone is an important part of a computer's sound system. Many programs, including instant voice messaging, voice-over-Internet Phones (VoIP), podcasts, and voice commands use speech as a source of commands or content.
Unless you're using a sound control interface intended for professional recording, you don't need to spend a lot of money on a microphone for your computer. All the fancy features that can make a microphone sound wonderful in a studio or on a stage can actually create bad sound if you plug the microphone into a computer.
It may look similar to the microphones you see on a stage, but the one plugged into your computer is there for just one reason: To make your voice easy to understand. For speech, the best place for a microphone is between six inches and a foot away from the speaker's mouth. If you're too close, the microphone distorts your voice; if it's too far away, the other noises in the room (including echoes from the walls and ceiling) overpower it. Yes, it's true that many singers work a lot closer, but their microphones are designed to eliminate the distortion known as proximity effect that occurs when an announcer or vocalist is too close (or emphasize it, if that's what they want). Look for a microphone with a table stand that can sit on the table close to your keyboard.