What is Time?
In music, time refers to the time signature, or meter of a song. Despite the large number of time signatures in existence, there are only 2 meters: duple and triple.
Duple: DOWN, up, DOWN, up
Triple: DOWN, up, up, DOWN, up up
All time signatures are derived from, or variations of, these 2 meters.
Why do we need it?
Listening to music and trying to identify the meter, time signature, and the rhythms are excellent ways of improving your sense of music. Later, think back and try to group certain rhythms with certain meters. You'll soon find patterns emerge, correlating these 3 elements. Unfortunately, this is something you need to experience for yourself. Listening to uncommon time signatures helps a lot. My personal favorite signature is 7/8
How does this help compose?
For example, 5/4, 7/8, 11/8, 13/8, 15/16
I think you get the picture. Any signature with an odd number of beats, will easily churn out an interesting rhythm. Unfortunately, 3/4 can't make this category, because it's pretty much over used, and, a bit slow to form an intriguing rhythm.