Candice Swanepoel applied to be a fit model for the brand.A fitting model (sometimes fit model) is a person who is used by a fashion designer or clothing manufacturer to check the fit, drape and visual appearance of a design on a ‘real’ human being, effectively acting as a live mannequin. 
So in other words, before the outfits will be put on the actual angels, they would put in on Candice to check the fit so that the angel doesn’t need to come in for 10 fittings before the show herself.
She worked with them for several years as a fit model before they decided to cast her in her first show in 2007 because the outfit she was being fitted for looked better on her than the model who was made to wear it. 
She then became a regular model for them and eventually an angel in 2010.