It (coincidentially or not) limits the performance of competing regional CDNs to that of Cloudflare, since 1.1.1.1 truncates the EDNS client subnet parameter, which means that the only location/region hint a third-party CDN DNS has to go by is Cloudflare's location, not the user's.
As an example, consider a user in Paris, and assume for the sake of this example that Cloudflare's closest POP is in Berlin. Assume that user visiting a site hosted by a French CDN, with POPs in both Paris and Berlin.
If that user tries to resolve cdn.mysite.fr via 1.1.1.1, they'd get a Berlin-based location, since the French CDN has no way of knowing that the user is actually closer to them, and Paris would have been the better option.