When you pick up multiple lashes at the same time and attach them to a natural lash the bond that is formed differs from the one you get in classic lashing. With classic lashing your synthetic lash sits on top of the natural lash whereas when you apply a volume fan, (your own rather than a premade one) as the adhesive is still in a liquid form the lashes wrap around the base lash forming a much stronger bond.
As you can see from the diagrams, with classic lashing a much smaller percentage of the outer edge of the natural lash is able to bond with the natural lashes and as the synthetic lash is heavier we dip a greater portion of it into the adhesive. This increases the weight on the lash and also reduces the flexibility. In comparison, volume lashing uses a very small amount of adhesive reducing the weight and increasing flexibility.
With Classic lashing you are limited to however many natural lashes are available but with volume you can safely double, quadruple etc the amount of lashes whilst still maintaining flexibility and comfort. This is especially great if you get a client in that has very few lashes. If you Classic lash them there's a good chance that they will be a little disappointed. Clients expect us to be miracle workers but with one on one application you are very limited with what you can provide them. Realistically, clients with sparse lashes just aren't going to be able to achieve a super full, glamorous look but with volume lashes you can get them as close as possible.
Also, following on from the diagrams above, as volume lashes wrap around the natural lash their retention tends to be a lot better and you can easily go 4 weeks between infills rather than 2-3. The difficulty comes when removing volume lashes. They are more difficult to remove than a classic lash because of the better bonding so an infill is more time consuming and tricky.