A carburetor is precision instrument so needs to be treated as such! It will never work properly if parts are worn or not to the correct specification.
The internals of carbs wear. When wear takes place there is a loss of performance and economy. This happens over a long period of time so is not always apparent.
Rebuilding carbs using a repair kit restores performance and economy and is often a more economical proposition that buying a number of single parts.
It is essential that carbs for a a multi cylinder bike all be rebuilt. The bike will never run properly if only some parts are replaced.
When carbs are rebuilt and and balanced the bike will start better run smoother and perform better than it has in years as everything is back to the OEM specification.
Don't ignore parts like air cut off valves carb pump diaphragms fuel pipe and filters. They all contribute to operating efficiency.