I tend to think of the affect of adding weight to a car the same as adding weight to you
If you are carrying more weight (say after a heavy Christmas) then it takes more effort to run, you tend to move/react slower and you can't jump as far
That is the same effect it will have to your car too, but unlike your Xmas pig-out, it isn't neccessarily unwanted on a car as it can be used to "tame" one end.
If you place more weight at the front then it will have the above affect at this end so may feel more consistent, drivable and have more grip at the front. The same reaction will happen to the rear if applied their instead.
I have found it helpful when I have tried set-up/suspension tweeks but need to go a little further than my spare parts can go or in particular conditions. It is also helpful to level a car out too if there are big jumps and you find you are nose diving etc.
Hope this helps.