There are several reasons for venting the room when spraying paint.
1. Overspray gets everywhere and sticks to surfaces causing damage to painted and unpainted surfaces
2. Too much overspray build up in the air can become quite dangerous and explosive. Even though today's modern materials have much less solvent and are less volatile they are still more dangerous when allowed to build to a high density.
3. Allowing too much overspray in the air will inhibit your ability to see causing poor quality repairs .
4. Higher concentration of toxins in the air can more easily cause health problems from exposure.
5. Venting is usually not that expensive on the hobbyist level and is usually only a small percentage of the cost of an overall restoration.