In my view, its to increase the contrast between the troops and the base. So that the nicely painted troops stand out and can be seen. Rather than blurring into the dark brown of the base.
These are some of my 10mm troops - but hopefully give the idea of what I mean.
Especially with smaller scales it helps to lighten and brighten the colours - there is very little painted surface for the light to bounce off so you need to help it, by making things a bit brighter than they naturally are. The same idea applies with the basing - you want to increase the contrast between the figures and the base. Your figures are fairly dark, so going light with the basing, will help the figures stand out.