nm L/R vs hdg
Sometimes particuarly in the tropical areas and with ISOL and SCT wx, repeated changes of heading to pick you way left and right around storms or TWR CU would be a pain in the arse for you and RT chatter, particularly if everyine else is doing it.
If seems much more sensible to ask for 20nm left and/or right of track and give yourself a corridor in which you can operate.
Sure in terminal areas, a heading works well somtimes, but in cruise at FLs, it seems better to give yourself "Horizontal Block Levels".
CS