Answer:
a source of revenue for the entity charging them
Explanation:
Processing fees are fees required to be paid for some service. Often, they are added to the nominal cost of the service. They may or may not be related to the actual cost associated with performing any "processing" associated with delivering the service.
Processing fees are often seen when there is 3rd-party involvement in a transaction.
For example, if you obtain a driver's license, there may be some cost associated with the license itself (say $50), and there may be an additional "processing fee" (say $10) charged by the entity delivering the licence to your hands. That entity may charge the processing fee because they receive no portion of the $50 you pay for the licence, but they still incur costs associated with its delivery. That $10 may or may not be related to actual costs.