You are probably right, i was really hoping for the provinces themselves to be buffed, i mean if the african and asian colonial provinces were so worthless, why would anyone ever have wanted them?
Well, let's see what Africa and Asia have in HOI3:
Rubber in Asia.
Uranium in Africa.
Gold in both. (Philippines and South Africa)
Oil in Asia.
Aluminum in China (seriously, Japan should grab it ASAP)
Black Soil in Asia (you do not want to let Japan combine a 100% conquered China with good agriculture techs and Black Soil in Vietnam)
Strategic effects for owning key portions of China and Asia that result in huge research bonuses that let Japan leap frog over US technological advantages
A resource whose name I cannot remember that grants a reduction in tropical attrition can be found in Asia.
Manpower in China. (even with no cores, it's decent for Japan)
Oil refineries in Asia
The Suez Canal in Africa
Various strategic places for naval bases.
Rares in Asia and India
Steel in Manchukuo
Now, the parts that were worthless in HOI3 were places like Italian North Africa (not economically important), Syria (not economically important), and a few other places.
And to be honest, some of these worthless places were, from the point of view of HOI3, worthless. Italy's colonies were famously less than awesome from an economic perspective. But when has economic viability ever stopped some countries from practicing imperialism?
Part of the problem of HOI3 is that stockpiling key resources is easy enough that the value of these areas is far less in the game than it was historically. I don't even need to invade DEI for oil as Japan even after the US embargoes me. I still have enough to drive my ships around for years. But that is not the same as saying these places were worthless in HOI3.