They are stronger (higher spring pressure).
And if your going to more aggressive cams, and or more boost, then you need more spring pressure.
The reason why I mention boost, is that it is acting against the valve closing.
The higher the boost, the more it tries to hold the valve open.
At 15 psi boost, there is over 18 pounds of force being exerted on the valve, trying to open it (or hold it open). At 20 psi it jumps to 25 pounds of force.
Force = pressure x area.
7M intake valve (stock 32mm) has 1.246 sq. in. of area.
The springs in your head are the same as the NA 7M uses, so they are boarder line at best, even at mild boost numbers.
Toyota didn't design the 7MGTE with the thought of:
"Oh, we know people are going to boost controls on these,
we better make them capable of taking 20 psi" !
No, they made it just good enough to get buy.
Add big cams, with more aggressive ramp angles to this mix, and you run into floating valves.
You don't need to have the same killer springs on a mild boosted engine as someone who is running 30+ psi.
So build for what your goals are.