One might liken modeling open calls to a huge swap meet where too many vendors have been invited, and the organizers said first come, first serve, and you witness pure carnage as people struggle for the best spots in the lot. Modeling open calls are similar, only probably slightly less violent. These are the places where dreams are made, and nervous breakdowns are had.
If you feel that you are a person prone to nervous breakdowns, modeling open calls may not be the way that you should plan on getting your career off the ground. Maybe your demeanor is more suited to just going on the auditions made by appointment. Part of being a success, however, means facing these kinds of challenges and following through, and don't sweat the other hundred people there all trying out for the same thing as you!
Modeling open calls could be a great way to meet and network with other people who want to do the same thing you do. This is part of building your career. You may come to a city not knowing anyone, but just by going out on a couple of modeling open calls, you could get to know someone who will help you become a success.