I think of it this way -
Trail is what makes the bike want to straighten out when you aren't giving the bike a steering input. The amount of weight on the front wheel makes a given amount of trail more or less effective at self centering the steering..........more weight on the front wheel makes the bike want to self center more and when you place a bag and weight up front nearly all that added weight acts on the front wheel and gives the steering more self centering.
So the idea is if you start with a lot of trail and then add cargo weight to the front it will have a high self centering force. This makes the bike more 'stable' and it will take more force to get the bike to turn.
So.............the idea is if one wants to carry weight on the front of the bike, and still wants nimble handling, the amount of trail should be reduced so that the added cargo weight, acting on the given amount of trail, doesn't make the bike more stable than is ideal.
My coffee hasn't fully kicked in yet so I hope I didn't just muddy the water with the above.
Have a great weekend.
dave