It makes no sense if all units here are equal.

It makes a lot of sense to add better unit.

Imagine a subway that only runs one train per line, such that on average each station only gets visited once an hour.

No one would use it.

Now add 10 more trains so trains stop every 5 minutes.

Many many people use it.

Not necessarily. Service frequency is a pretty significant factor influencing people's decision whether to use public transport.

A lot more people will start using a service if it's frequent enough that they don't need to plan around it.

That depends why units are underutilized, though. If the train + bus system doesn't get you close to where you need to go, it might not be worth your time to use. If it takes too long, again, won't be worth the time. If they can't get you to work on time or stop running before you are off work? Again, not worth your time to use them.

In all of these cases, a good answer is to have more convenient public transport - and that usually means adding more units.

Other problems have other solutions, like adding tolls to congested areas, subsidizing public transportation so the fares are low enough for people to afford or choose it over driving, or increasing private vehicle ownership costs.