Differences between 3/8, 6/8, 9/8 and 12/8?

I'm a little bit confused about those time signatures. I'm aware of the usual strong/weak patterns for those, which are:
3/8: |Sww|
6/8: |SwwSww|
9/8: |SwwSwwSww|
12/8: |SwwSwwSwwSww|

So my question is why is 6/8, 9/8 and 12/8 used when the Sww pattern is repeated anyways? After all 6/8 seems to be 2 bars of 3/8 time in one bar, 9/8 seems to be 3 bars of 3/8 time in one bar and 12/8 seems to be 4 bars of 3/8 in one bar/ 2 bars of 6/8 time in one bar. I hope you get my point.
Is there something special with 6/8 compared to 3/8, with 9/8 compared to 6/8 or 3/8 or with 12/8 compared to 9/8, 6/8 or 3/8? Or is it a matter of making things look nice on the sheet music (I doubt it)?

6/8= same with 3/4 (one two three) but 6/8 is (1/2(one) 1/2(two) 1/2(three) + 1/2(one) 1/2(two) 1/2(three)
the half note count one note and in 6/8 is same with 3/4 + 3/4 =6/8

other pattern is same likes that just count it that only exsample
info http://en.wikipedia.org/wiki/Time_signature

All of the 3/8 6/8 9/8 12/8 time signatures are basically the same thing. The only difference is more notes might fit into one measure than others, but that does not affect the music.

Composers may use 6/8 oppose to 3/8 because they might have very long notes, for example a half note does not fit into a meause of 3/8, but it does fit into a measure of 6/8

Very simple: on the 3,6,9,12/8 time signatures, one "beat" is a dotted 4th! So 3/8 has only 1 accented beat, 6/8 has 2, etc. They are like the "triplet" version of 1/4 (not really used because when it's binary it lacks substance), 2/4, 3/4, 4/4. I hope that helps!