When we created Metric Sources, we supported arbitrary SQL queries to maximize flexibility. We've now added support for directly pointing to tables for when that's what you want.
Using tables directly is simpler and improves performance. We can be pick just the columns we need to operate on even when the table is very wide. Complex filters can be applied efficiently without your SQL engine first trying to materialize a CTE.
An added perk with using Tables as a Metric Source is being able to use formulae. You can apply simple SQL transforms to columns (e.g. convert from cents to dollars by dividing by 100) or alias them to make them more discoverable.