John Siegel writes:
I understand speed controllers but not why you would want one on a
router. For a handheld router, any bit large enough to require
reducing the speed is probably too large to use safely hand held.

Speed controllers are also used when small bits need to be run slower,
such as with woods prone to burning or delicate or intricate cuts
where you can't move the router fast enough to avoid burning.
Especially with piloted bits instead of bearing bits.

At least, that's what I use my speed controller for.