I am not familiar with that combo, but generally speaking, a line out is just for interfacing to other line level gear. This could be a line input on a recording device, an effects box or a power amp. You could also for instance plug it into another amp. Lots of amp heads and combos have "pre out" followed by a "power amp in" By using a power amp in on a 2nd amp you would be bypassing the preamp/eq of that 2nd amp, simply utilizing the power to bring up your volume.
The me20b is simply an effect pedal, so it can be viewed as a preamp itself.