In deploying ARA, Google discovered that there are a great many intermediaries who are incapable of running JavaScript and many others that prefer not to. There's a lot of things like tracking pixels, with the HTTP interactions to load those pixels generating measurement signals as a side effect.

Adding HTTP header fields to those requests that trigger the equivalent of the APIs might make the entire system easier for these systems to consume.

There are some serious difficulties involved with this mode of interaction. Responses will contain instructions to the browser to save impressions or measure conversions. New requests will be needed in order to deliver the results of those interactions. That was true for ARA, but the delays meant that new requests were inevitable. Here, the new interactions would need to be synthesized somehow. Using a redirect is one possible option.