> what is the benefit of "composing" queries via this library instead of just running them separately in a single connection?

I think the link is pretty clear: it combines and batches queries while providing a type-safe interface for the returned results. It further provides intellisense and unit testing features that are tedious and error-prone to achieve in other ways.

Of course you can do this all by hand, or manually batch your queries to avoid round trips, but why would you want to?