(This part of) Akamaiks value prop rests on two things: they have servers all over the world, and they can figure out which of those are both functioning well and (network-wise) close to you. The former comes from infrastructure investment and management. The latter comes from collecting and processing data from all over the (network-wise) world, quickly. That last part involves building and updating a map of actual and potential internet traffic — not quite all of it, but for everywhere that your (prospective Akamai customer ‘you’) customers might be. Doing _that_ without owning ~all of the BGP routers in the world plus ~all of the local ISPs in the world involves a fair bit of probing, data gathering, and a bunch of math.
It’s likely that those headers don’t all get added all the time, for all the Akamai traffic, but instead are added selectively for key parts of the mapping process.
(Disclosure: I worked for Akamai Way Back When, but left the company many years ago.)