Multitracking is a method of recording which allows you to record separate sound sources simultaneously or individually to create a whole piece. This is useful when, for example, only one part of the song wants to be heard, such as the guitar. By muting all the other instruments you can hear just the guitar whereas if all of the instruments were recorded onto the same channel they would be inseparable and there would be no way to extract the guitar with precision to isolate it. This is also beneficial when it comes to mixing the track as it is easy to retain complete control over sculpting each individual component of the song.