If you can take off shorter than the end, you may taxi full length if for some reason you won't be allowed to take off immediately (ATC slot time, queue, special weather). Sometimes your take-off performance calculations tables are missing for intersection take-offs and you can only go full length. Sometimes pilots will only use full length if available out of personal preference. The longer the take-off run available, the lower the power you can use increasing engine life. Nowadays heavy engine maintenance is very often done 'on condition' which means leave it there on the wing until something goes wrong, so nursing the engines can increase lifespan. If you are running late, sometimes you want to get airborne ASAP, so a shorter take-off is preferable. It's largely down to the pilot on the day.