This option is related to --time-limit, but stops after processing X bytes. For example, I often need to free up a certain amount of space on a disk and want git-annex move to stop after it has copied a specified amount of data:

git annex move --to otherdisk --byte-limit 3G
git annex drop --auto --byte-limit 500m
git annex get --byte-limit 500m

I've been using some annex.diskreserve trickery now and then to accomplish this, but it's a bit cumbersome and also not a very precise way to specify the amount I want to copy/move/get. The last example would also be a useful command to limit the traffic when I'm connecting via mobile – get as much as possible, but don't blow the mobile quota.