When friends ask for a favor they often suggest to pay you for your services.
However usually their budget is under market rate. They give you some money as compensation.
The problem with this is that because they are paying you they’ll expect you to be available and work hard for you. While you as the service provider don’t want to give it priority over your actual clients.
This causes friction on the relationship. They expect you to work hard and spent extra time finetuning it.
So either charge a full price so you can give it your all and you don’t have to down prioritize is over other commitments or do it for free so they can’t claim your time.