To add to David's reply:
Reap[Sow[1, {x, x}]; Sow[2, y]; Sow[3, x]; 2 + 2]
{4, {{1, 1, 3}, {2}}}
1 is 'sown' twice with tag x, then 2 is sown once with tag y, and then 3 is sown with tag 'x'. Now for each tag it is 'reaped'. That is the reason for the list of the lists...
