[fmap]

Tactics are very useful when developing proofs rather than dependently typed programs (unless you are using very precise types, as is common in e.g. the Coq program package).

The point is that in a proof you are usually not interested in the precise shape or computation behavior of the underlying proof term and this allows you to use far more aggressive automation than what you are doing with a typical macro. Likewise, you don't really want to see the proof term in the first place. Think about how long chains of equational reasoning are encoded in type theory - this is just noise that detracts from writing your proofs.

As for homoiconicity, I think this is orthogonal to a tactic language. Both lean's tactics and template Coq allow you to reify the syntax of the type theory in order to write automation tactics, but this is separate from the proof search aspects of tactics.