Benefits of using an invite code in Ingress

Quick answer: Apparently nothing.

The recruiter obviously gets something (a point on the Recruiter badge, as soon as the recipient reaches Level 3), but that's not the point of this question.

Years ago Ingress was invite-only, you needed a code to join. Even then it gave no new items or anything, and AFAIK it still does not.

Invites are left in the game, possibly because that's cheaper and easier than removing them. And they can still be useful, you talk about the game with your mate and they're interested, you can send them an invite and the email is a reminder of the talk and the game's name.