I think there are basically two reasons why we would want centralized connection management in the platform:

• Consistent handling of overrides of resolving names (domains), e.g., mapping localhost to the loopback address, as discussed at Incorperate let-localhost-be-localhost whatwg/fetch#1118.
• Consistent requirements on partitioning of all aspects of the connection. https://fetch.spec.whatwg.org/#network-partition-keys in Fetch is now used for connections and the HTTP cache, but it should also apply to the DNS cache, etc. Having those kind of requirements once would be beneficial. Also in case we ever decide to adjust them.

(See also w3c/webtransport#128.)