OPS 1.375
In-flight fuel management
(a) in-flight fuel checks
An operator shall establish a procedure to ensure that in-flight fuel checks and fuel management are carried out according to the following criteria:
1. a commander must ensure that fuel checks are carried out in-flight at regular intervals. The usable remaining fuel must be recorded and evaluated to:
(i) compare actual consumption with planned consumption;
(ii) check that the usable remaining fuel is sufficient to complete the flight, in accordance with paragraph (b) “In-flight fuel management” below; and
(iii) determine the expected usable fuel remaining on arrival at the destination aerodrome;
2. the relevant fuel data must be recorded
Aside From the reasons given in previous posts, this would be a reason as well. Depending on aircraft type and FMS the FMS prediction may not always be accurate. Step climbs you can not program etc.