Businesses, like your affiliate, grow by getting new customers.
Your website can be an engine that converts visitors into new members. A website acts as a salesperson for your CrossFit box.
Visitors come to the site through different channels. Perhaps a friend told them about your box. Maybe they googled you, or saw one of your advertisements. Either way you want the website to be aimed at these prospective members so that they come in to try out your box and become your next member. Your website needs to be the salesperson.
The most important thing to do is make the website design welcoming and the sign up process really easy.
Consider your face-to-face interaction with a prospective member. Think about the questions they ask and the answers you provide to convince them to join.
Those answers become the content on your website. From the homepage to the signup form your website acts as the salesperson guiding a new visitor to the eventual membership and to them achieving elite, functional fitness.
If your website converts people well it stops being a cost and starts being revenue positive.