The postcode doesn't tell the whole story. But what you can do is use an IP geolocation service which should narrow down your location enough, so that typing in the entire address is no longer necessary.
I.e. using something like https://ipinfo.io/json and then typing in a full postcode and street name + number should work well in most cases.
What if I order something on the road and want it delivered to my home? Or what if I want to order something over mobile? My mobile IP is often 1500km away from where I live.
Autofill solves all of that with an implementation cost that approaches zero.
IP geolocation is increasingly not useful for anything, especially for mobile users. The best it can do is give you the correct country and maybe get you in the right region.