It’s important to understand the difference between Mark Ups and Interest.
Mark Up required to personalized the rates you are sending to the channels as explained here
Interest is a real commission charged by the OTA from you and it can be equal to Mark-up, or higher. It can be lower as well, but it means losing your profit.
It will be easier to understand it by example. Due to Rate parity (price parity) conditions on many OTA you have to provide the same rates for all players on the market, as a result it comes to higher revenue for the hotelier from the OTA with less commissions.
The main purpose of entering Interest is to get proper Financial reports from Effective Tours which you can use for future analytics.