LarryJ

The problem on a very short flight like YYZ-ORD is that you're limited by your landing weight. You can't take so much extra fuel that you'll be too heavy to land if you don't receive significant airborne delays. You can only take fuel up to your landing weight limit plus the planned fuel burn. Anything over that makes you illegal to depart.

For example, a 737-900ER has a max landing weight limit of 157,300 pounds. If your planned fuel burn is only 4,000 pounds then your max takeoff weight is restricted to 161,300 even though the airplane is capable of taking off at 187,700 pounds. That's 26,400 pounds of additional fuel that the airplane could carry if it were planning on a longer flight.

I've had a several flights over the years where we were planned for a longer weather avoidance route but by the time we actually got there the weather had moved or dissipated and we were able to fly a much more direct route. The result was that we were too heavy to land and had to descend early and hold to burn off the excess weight.

Once airborne, if you receive delays in excess of your holding fuel then it's often a good idea to divert early so you can be refueled and ready to depart again. No reason to spend a long time in a hold, even if you have the fuel to do so, if you don't have an expectation of being able to reach the destination when the hold is lifted.