Most of the music that people play on guitars needs to be very precise. A human is a pretty rhytmical creature – the heart is beating and fingers keep strumming on the table. If you take a longer run of walk then you will feel how you enter the rhytm. In music it is very similar.
Accuracy that you need in the so called rhytm music styles (in some languages pop&jazz are called the rhytm music) is almost machine-like. Let’s say that the tempo is 120 bpm (beats per minute). If you play it some percents slower, then people who are used to this style can already notice that something is wrong. Not to mention if you miss the beat of a band. That’s when you need a metronome. First you need it for recognising the tempo and then for keeping it. Those tools can be found in many programs and as a hardware boxes. But also there are useful flash plugins as this.
Besides recognition you can use this tool to train your tapping accuracy. Tap a tempo and try to keep it for a while. After some practise you will be able to avoid tempo changes. You will notice soon that some tempos are easier to keep than others. That is why the aspiring guitarists tend often to slip into the same tempo.
There are different effective practise plans that you can follow but one thing is sure – you have to include some practise with a metronome. Even in classical music where you often have to change the tempo. In this case you still can practise with a click and you should. Because changing a tempo is possible only if you compare it to something. Then you will be also able to come back to the initial tempo. Here you find a simple metronome.
And here is a bit more functional metronome.