Creating a custom field is a great way to ask new leads specific questions about their current fitness experience or other pertinent information that could be useful to you to know about them.
If you're creating a nutrition program at your gym for example, these custom questions may be useful to ask incoming clients specific questions on their current knowledge or expectations for the program.
Or, maybe you're running a special clinic or competition and you want to get shirt sizes and have them sign a waive. Another useful tip is to create a "Employment Application form" and use the "Upload field" to have them upload a resume!
The point is, the options are endless and are only hindered by your own creativity on how you wish you use them.
*Note: Want to Trigger a workflow based on how a lead or client answers a specific question on a form? This will allow you to completely customize a new client journey based on how they answer! Here's a help article on how to accomplish this: http://help.pushpress.com/en/articles/6197818-grow-create-a-customized-lead-nurture-workflow-using-custom-field-responses