In the situation above, the person writing the letter is explaining what consequences the tenant will face if he doesn't pay his rent by a certain date. This is a kind of threat. People can get really angry, frightened, or offended if they feel like they're being threatened in a direct way. For example, writing the sentence above this way would probably start an argument:
If you don't pay us by January 15th, we're going go charge you a $150 penalty.
This version is too direct for communicating with a customer or tenant. Instead, the person in the example explained the consequence of not paying in a more indirect way:
Here are some other examples:
If it isn't completed by the end of the month, someone is going to get fired.
If all the steps are not followed in the correct order, the results will be ruined.