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this option is used to put your torrent data in another folder once your download is 100% completed

this creates a copy of your file onto the destination and then deletes the original.

since my target location is on the same drive this results in a serious performance hit.
I read from same drive & write to the same :
1>  the drive becomes quite busy at that time (disk queue length:5...) and may impact other running processes (other active torrents etc)
       
2>  you need alot more "temporary" disk space.
      for "simple" torrents this should not be a big problem
      however when using large torrents (>50 GB) with -worst case scenario- alot of small files this is just plain deadly.

this may be the reason of the issues i had recently since my torrent drive was nearly full
like unresponsiveness and missing data files.
and yes,  i DO use the "preallocate space" option.

i there a reason not to use a move functionality ?

i tested the "set location" option an a 15GB seeding torrent. this option does use a "move"

i cannot set it empty so i set the "save file to location" identical to the "keep incomplete torrents in" folder. hopefully that will disable it

Libtorrent handles the moving of payloads (http://libtorrent.org/manual.html#storage-interface) and it 'moves' the payload in different ways depending on whether the payload is complete or not.

Incomplete requires the pieces to remain intact during the "move_storage" operation, complete jobs can be moved using the operating system file manager.

i cannot set it empty so i set the "save file to location" identical to the "keep incomplete torrents in" folder. hopefully that will disable it

Just uncheck the box of "Move to" location to have it empty.