When to use "index: true" exactly

[unonweb]

Hi,
according to the docs...

Set this field to true if your users will perform queries on this field's data often.

... and this post:

So if I understand correctly, if I had a collection of posts (image, text, date, few boolean values) there isn't much point indexing them. Since we need the entire collection and don't query for single fields or values

So we use it if we query a single field often.
Well, does that mean in the end, that it's recommended to use index: true when we apply where queries to a field?
Or is it a GraphQL thing?