Why bother with projections if you are going to drop fields in application code anyway?

Short answer

Projection cuts what the server sends over the wire and what your process has to deserialise. It can also make a query covered, meaning MongoDB answers it from the index without fetching the document.

Why?

Inclusion and exclusion cannot be mixed, except excluding _id on an include list. _id is returned unless you set _id: 0. Returning a 2MB user profile to render a name in a list is a common production bug.

Example

db.users.find(
  { city: "Bangalore" },
  { name: 1, email: 1 }
)

Interview tip

Tie projection to payload size and covered queries, not only to 'cleaner JSON'.

Common mistake

Projecting in the app after find({}). The damage is already done on the wire.