Number of visits allows you to be more specific on when, what, how, and with who the visit was with than milestone visits!
You can specify the status of the visit; late cancel, no show, check-in, absent etc.!
These behaviors can be tracked over a time frame as well, and you can filter on instructor, credit used, membership used, and class time.
Creating an autocampaign to upsell someone once they sign in three times using class pass within the last month.
Creating an autocampaign based on clients who no showed their first ever visit.