Plinths for Monitor Audio Silver 8

I purchased a pair of Monitor Audio Silver 8's a couple of months ago and they sound great.
I have what is definitely a newbie question.
They came fitted with nice heavy metal plinths and then in separate boxes are two more plinths.
The ones on the speakers are flat and raise the speaker a couple of inches from the floor. The extra ones are slightly rounded and would seem to raise the speaker up just a bit more.
What is the reason for the two styles and how do you determine what is best for a particular use.
I am using the soft silicone feet on a hardwood floor.
Thanks