This is going to be a real rookie question, so please be gentle.

I've never used router template guide bushings before, but I am about to.

To install this bed rail hardware, I need to make a 5/8" mortise for the
slotted portion and then two 1/4" mortises right in the center of that
mortise to accept the hooks. My testing shows that a single pass with a
5/8" bit makes a nice snug mortise for the hardware.

http://www.cshardware.com/15-74-5.html

It seems to me that if I make a jig to accept the bushing for the 5/8"
bit, then all I need to do is swap out the bit and the same jig can be
used for both mortises. The 1/4" bit will be centered in the 5/8" mortise
"by default". That seems pretty straight forward.

So assuming that I am OK with using that technique, here's my question:

Why are there so many different sized bushings? Why not have just 1 bushing
to accept the largest bit and then make all jigs to work with that one
bushing? All bits, regardless of their size, will be centered in the bushing,
so why are there multiple bushings that match the bits when its the *jig* that
determines where the bit will contact the wood?

I'm sure I'm missing something simple due to lack of experience.

Thanks!