Perception. All residents are ultimately paying for common expenses. It takes a certain amount of income to operate and residents are the source of that income, so they will be charged in one form or another.
For convenience, let's say you have two communities where the common expenses divided by the number of units works out to $100. Now let's say that identical apartments in each community should rent for $1,000 (which includes the common fees).
One manager is going to advertise the unit for $1,000, rolling the common expenses into the rent. The other manager advertises the rent as $900, and then adds a separate line item of $100 for the common expenses. Either way, the resident will be charged $1,000.
When both properties advertise, which one will generate higher interest? If a potential resident calls the "higher-priced" property, it can be explained that the competition is charging fees that this community has already included. But that conversation never happens if there is no call.
Anywhere there is market competition, it is important to advertise apples-to-apples as much as possible.