Take the verb to wait, for example. Speakers and writers of the English language will often use a preposition such as for or on with this verb. The conventions of speech are more lax, and so often such prepositions can be and are used interchangeably. Such is not the case, however, with written English. It is a different animal!
Careful writers understand the difference between waiting for and waiting on. Waiting for is usually correct, unless the sentence treats the actions of employees interacting with customers. In that case, waiting on is appropriate.
Consider the example sentence in the graphic above. At first glance, many writers may not think that sentence needs any improvement. However, no customers are being attended. That makes waiting on a less effective choice. Waiting for is more effective. Here's how the revised sentence would look.