Why we need a full featured indexer?

Not really: for generic indexer, you can be satisfied with raw json output, but with custom indexer, you would be more confortable with shortcuts.
It’s a shame I’m incapable of setting up what elois did with hasura (Subsquid (ancienne Hydra): indexer de blockchain substrate exposant des API GraphQL). I’m too bad at docker :cry:
Did you try it before it went down?