My Layla D/A converter (which I suspect is very much like whatever echo unit you have) has a left & right input assignable to whatever tracks I "arm" (record on)
So, the left input feeds tracks 1, 3, 5 & 7 and the right input feeds 2, 4, 6, & 8. This means that even with only 2 inputs I can record at least 8 tracks of digital audio (technically I could record more than 8 tracks - but would lose individual control coming out of the converter.
The 8 outs let me have individual control of the output of 8 recorded tracks. Naturally I want as much seperate control of the output as possible.
Actually, I have 8 in and 10 out - but since the majoity of the stuff I do is one track at a time (ie: a guitar track of a vocal track) I rarely need all 8 inputs.
I suspect you gear works much the same way.