From The Wiktionary:

(Noun) A device, containing an inverted pendulum, used to mark time by means of regular ticks at adjustable intervals; an electronic equivalent that emits flashes
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The metronome is a product of Amsterdam invented by Dietrich Nikolaus Winkel in 1812. In 1816, Johann Malzal copied the construction of Mr. Winkel’s metronome and got a patent on "his" new invention. Whenever the abbreviation M.M. is used in front of a tempo marking, it stands for "Malzel's Metronome." In our modern era, we use BPM now instead of M.M. (which stands for “beats per minute.")

The argument against using metronomes is that critics complain it stifles creativity. Franz Liszt was one composer who did not advocate the use of the metronome, stating the melody was more important than the rhythm.

Musicians in modern recording studios use what is called track recording. Instead of everyone in the band getting together and everyone recording together hoping it works out okay, musicians record their parts separately. In the end, all of the tracks are put together. For this reason, the metronome is an absolute necessity for recording.

On the movie set, for cartoons and for computer-drawn films, a "click track" is used, which is nothing more than a metronome that is used to keep all the parts synchronized, especially when it comes to the music being in-line with what is seen.

In solo practice and recording, the choice to use a metronome is entirely up to the musician. It is generally recommended by expert musicians to not use the metronome all the time while practicing, but it can be a valuable tool. It is especially useful to stop the phenomena that happens to all of us to start our piece slower and end up faster. It is also a good tool to use to learn control of a piece. It is well-known that a piece when played rhythmically correct is easier to play and sounds more pleasing to the ear.