Again, tailpipe exhaust is only part of the story with emissions. Another major source of pollutants from engines is fuel fumes evaporating from the fuel system and gas tank, both when the engine is off and on. As long as the gas tank has a trace of fuel in it, it emits vapors, a source of pollution. The Evap system traps all these fuel vapors to be consumed by the engine reducing pollution and improving MPGs. Evap being closed venting also reduces moisture entering the gas tank, keeping your fuel system cleaner and improves longevity and reliability. Its not just the gas tank either, for carbed engines Evap is also closed venting for the carb bowl, which also keeps the carb cleaner and reduces gunk buildup. However, with older cars the only way to test Evap is visual, only other way is to lock the car in a air tight test chamber for two weeks. Modern OBD2 cars have built in sensors to test their enhanced evap systems. This is just the tip of the iceberg.
PCV is also similar to Evap, it closed vents the crankcase keeping moisture out. The same moisture that forms acid in motor oil. However, it only works if you keep the breather to the aircleaner. This also lowers under hood fume buildup thus slowing engine bay degradation. Again the only practal test is visual.
Then there are the snorkel aircleaners, the main function is to trap the fuel cloud above the carb inlet formed by reversion. You really don't want a fuel cloud floating in the engine bay either. A well designed snorkel should plump cool outside air into the engine as well as using Thermac to speed up engine warmup and reducing wear. PCV also has reversion which is why the PCV breather is routed into the stock snorkel aircleaner to be trapped and burned. Once more the only practical test is a visual inspection.
In short, low emissions is much more than just cleaning up the exhaust pipe. It is also cleaning up all the powertrain venting which has the beneficial side effect of increasing reliability and longevity of components. Its not just burning cleaner, but also venting cleaner and even sitting around turned off cleaner.
Because many of these systems are hard to test in the field at inspection stations is why the government lab tests engine packages submitted to them for certification. Every entire engine/powertrain configuration, including their low emission package is certified by the EPA or CARB. One reason they don't like tampering is because many modifications can't be tested at inspection stations and the average Joe can't afford a certification test that costs millions of dollars and subjects the test vehicle to 100k miles of testing. Its not just certifying that a car burns and vents clean, but also that is does so for many miles without degradation. Instead they mandate everything must be in place and working according to factory specs as a matter of practicality for inspections. OEMs have far larger R&D budgets than the aftermarket let alone shade tree mechanics, they know what they are doing.
