Difference between selectGraphQLResultFromTable and context.pgClient.query
<!-- Before opening an issue: if you use `npm` try running `yarn` instead; if
      you run `yarn`, please try running `npx yarn-deduplicate && yarn`. -->

<!-- You can also chat with us on Discord: http://discord.gg/graphile -->

### Summary

<!-- Please detail the issue you'd like to discuss. -->

I've read the documentation of `selectGraphQLResultFromTable` that are found here https://www.graphile.org/postgraphile/make-extend-schema-plugin/#the-selectgraphqlresultfromtable-helper and I still cannot fully grasp the difference between `selectGraphQLResultFromTable` and `context.pgClient.query`. It's also not clear to me when one should be used instead of the other.

Can someone please dumb it down more for me?

### Additional context

<!-- Please provide additional context such as environment versions (including
      operating system, Node, PostgreSQL, Redis, etc. as relevant), software
      versions (from `yarn.lock`) or similar, plugins in use, database schema
      if relevant, bundler (if any). -->
