Earlier today I was discussing a modeling portfolio with a prospective client. She wants to get into modeling, and asked me about shooting a beginner's portfolio. She wasn't familiar with the concept of a comp card. Here's what I told her...
For a beginning model, a comp card is the essential first step. We shoot 3-4 different looks, and you use the comp card to find a print agent. Once you've met an agent, they will help develop your portfolio by sending you out to various photographers. Usually you pay these photographers, but sometimes the arrangement is "work for prints"- both the model and photographer are building their book, so nobody gets paid (stylist and makeup artists also work for prints). Usually everyone involved is inexperienced, so the results can vary wildly. Sometimes you can get a great shot, and often you leave with nothing usable. It's essential to have a professional comp card to begin with, otherwise the agents won't call you in. If you try to put together a comp card with free or inexpensive shoots, chances are the results won't be good enough to interest them. Once you get an agent, trust them to help build your book. Of course, the photos for your comp card will be in your portfolio as well, but shooting a full portfolio right off the bat doesn't make sense. A portfolio is built up over time with many different photographers. Stick with the comp card initially.
By the way, comp cards are not just for models- actors should have one as well. Anyone interested in pursuing print work should have a comp card. For actors, a portfolio is unnecessary- just a comp card and headshot is all you need. For fashion models, a portfolio is a great tool in addition to the comp card.