This is usefull if we have 3 collections
"foo" (100m rows - we want split it to two servers)
"bar" (not big, but many q/s to it)
"baz" (not big, but also many q/s to it)

We have two servers and descide to setup sharding

So if we shard collection "foo" to two servers, this will help for our case
But also if we can move collection "baz" to another server it will be great

And "mongos" just forward our queries to "baz" to another server