Buying these saves time over a rebuild of old,worn carburetors as most 50+ year old Zenith carb's have worn housing in the body and push-rods. All of these parts are not readily available so even a rebuild kit may not provide an effective repair on an original caburetor.
If you're a purist these carbs can provide a good parts source on an original.
Most original carb's have a damaged or worn accelerator pump internally which reduces operation and efficiency of the carb especially on accelleration.
Replacing an original with these reproduction items provides a good alternative and a smoother start, better idle and driving. As good, if not, better than an original part.