spidey, it has to do with grouping.
first you set tempo.. (bpm)
then you group sets of beats into groups, called measures. In musical situations where there are many parts, and many measures, often you have to say " go the 15th measure"
So when someone says I need a beat to last an entire measure that beat or note is called "whole note" because it takes the whole measure. If you have 4 beats in a measure, it's 4 quarter notes. With complicated music and many musicians doing different things, it's easier to find your way deep into a piece of music if the measures are consistent in length and you can say "go to the 16th measure." which would be the same as saying the 72'nd beat. (assuming each measure is 4 beats long)...
that, and 1-2-1-2-1-2, seems so sudden to repeat, 1-2-3-4 has more depth, but 1-2-3-4-5-6-7-8 may seem like tooo much.
on a side note.. 4/4 is relative. the first 4 is most important, as that dictates how many counts per measure.. the second four simply describes how each count is written.. if its 4, you write it as a quarter note.. if it's an 8, you write it as 8th notes. Theoretically, 4/8 and 4/4 should feel the same. It's only when you relate those counts to tempo that it matters.
1-2-3-4 is a comprimise of countability, even looping against a metronome, and ease in playability. It's easier to play a complex melody across 16 4 beat measures than having to worry about each subsequent measure being a different count and presenting a different, and sporatic sense of where the downbeat will fall.
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