As TWB says the weight limits will be there for legal reasons. The reason is that the more weight you put in the top box the lighter your steering will become - boxes generally are behind the rear wheel and if you think of it like a see-saw - put more weight on one end and the other end rises - you'll realise that more weight in the box means less weight on the front wheel.
In practice you can put quite a bit of stuff in there before it makes any difference and probably 20+ kg before you have to compensate with your riding style. It does depend on your bike - weight distribution and how much power do have an effect but I've carried pillions on rear racks before and still been able to steer (just).
Sometimes the limit isn't the handling problem but the rack engineering - either the mounts are not strong enough or the rack is plastic or something similar. Usually though if you can lift the rear of the bike by the rack it'll be strong enough to take as much as you'd want to put on it.
Couple of pics below of the amount of stuff I've loaded onto racks for long trips - just under 20kg in the first pic and god knows how much in the second very ancient pic (well over 20kg anyway)
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