An agency holds the accounts — a DSP seat, a direct relationship with a publisher's sales team, a retail media platform — and buys through them for you. You brief them on the outcome and the budget; they choose the ad spots, negotiate the rate, run the campaign and report on it. You are paying for the accounts they already have and the people who know how to use them, which is why an agency is usually the answer when the buy is bigger than your team.
Buy direct when you know where you want to advertise and the seller will sell to you — a self-serve platform, a newsletter with a rate card, a site with an advertise page. Brief an agency when the buy spans more sellers than you can manage, when the doors you need require a seat you do not have, or when the minimum spends are above what a seller will take from a stranger. The doors on each listing here tell you which of those you are facing before you ask anyone.
Each row is a company that buys advertising on behalf of clients, recorded with its own name and its own site. Nobody paid to be on this page and the order is alphabetical, not a ranking — this directory sells no placement and takes no commission, so there is nothing here that could move an agency up. It is a list of who exists, not advice about who is good: the judgement about which one to brief is yours, and nothing on this page is a recommendation.