The density only is important in certain circumstances. Since spray foam insulation is generally sold in a specified thickness such as 3 or 5 inches or some other thickness, applicators like lower densities as this generally means material savings. When sound proofing, lower densities are preferred as the spray foam is softer when the density is lower. Softer spray foam insulation absorbs sound better. On the other hand, roof foam applied externally on flat roofs must have a certain compressive strength (> 45 psi) which is achieved at a density of about 2.8 lbs. Just for reference: open cell foam usually comes in around 0.5 to 0.6 lbs. per cubic foot, interior closed cell around 1.9 lbs. per cubic foot.