I get asked from time to time what is the point of time signatures? The thinking is why use this: 4/4 G / / / | D / / / | Em / / / | C / / / |, instead of G / / / D / / / Em / / / C / / /? (Note the / represents a beat) Why break apart line and count 1 2 3 4 1 2 3 4... rather than 1 2 3 4 5 6 7 8...?
Time signatures, for those who don't know, are the fraction looking things found in the beginning of measures. The top represents the number of beats in a measure. The bottom represents the note that gets the beat. For example, a time signature of 4/4 means that there are 4 beats in a measure and the quarter note gets the beat. 6/8 means that there are 6 beats in a measure and the eighth note gets the beat.
First there are is a practical application for time signatures. Take the typical rock song, about 3:30 minutes with a bpm of about 110. If you were counting 1 2 3 4 5 6 7 8..., you would eventually get to 330 331 332 333..., and that would just get to complicated. If you get lost, it is easier to find the first beat of a new measure to right yourself, rather than determining if you are on beat 323 or beat 330.
Of course, if you were just sitting in front of the music, playing along, the counting aspect probably wouldn't be much of a concern. You could just read the beats, like a person reading a book moving, from one beat to the next. I find the book analogy apt, music without a time signature is like text without punctuation. The time signature shapes phrases, gives a song structure, and allows one to analyze and make sense of a song.
Most importantly, time signatures each have their own connotation, and tell one how to play a song. For example, when you see 4/4 you know that the 2nd and 4th beat gets emphasized. Imagine yourself in a big arena, the band is rocking, and you are clapping over you head. You are most likely clapping on 2 and 4, that is just the nature of 4/4. In ¾, the first beat usually gets the emphasis, like a waltz: ONE two three, ONE two three.
Lets just look at 12 beats. Without time signatures they would look like this:
/ / / / / / / / / / / /.
A musician would not know where to put the emphasis. If those beats are in 4/4, they would look like this:
| / X / X | / X / X | / X / X|
(Note that X means that the beat is emphasized, played like one TWO three FOUR.)
If those beats were in ¾, they would look like this:
| X / / | X / / | X / / | X / / |,
ONE two three.
Just think count in you head emphasizing the capitalized numbers and you will get the feel, and notice the difference.
This is just a simple explanation, using the two most common time signatures. I have tried to illustrate how time signatures shape a song, and why they are necessary. Maybe I will get to an explanation of complex time signatures, and their various uses. Feel free to mention anything in the comments that you feel that I missed, or have gotten wrong.