purpose of local.slaves and local.me collections

[Zardosht Kasheff]

Hello all,

What is the purpose of the "local.slaves" collection in replica sets?
I know what it does, it saves the OpTime that slaves have synced to.
Is it there just for diagnostic purposes, or does some piece of the
internal system actually use it? I could not find any code that
actually used it, which makes me suspect it is there for just
diagnostic purposes. If that is the case, why would one examine that
over rs.status()?

Also, what is the purpose for having the local.me collection be
written to by the oplogreader? I could not find any code using that
either.

Thanks
-Zardosht