LR - two reasons come to mind straightaway:
NB Assuming the gauge is accurate!!
1) There is a lower limit on the 'freeze point' of aviation fuel (typically -47deg C) and this could (rarely) be a problem.
2) It is a good clue as to whether the wings will accumate ice on a turn-round due to condensation on a cold-soaked surface
I guess on Concorde (RIP) there was a heating consideration?