30W is fantastic for most engraving that isn't deep, but anytime you want to go deep or cut thin metals (e.g. aluminum foil, pennies, etc) you will usually want a smaller lens and 50W or higher power.
The difference in power is important when trying to do deep engraving, coins, cutting very thin metals, etc. It is also important as lenses get wider. Essentially you need much more power if you want to go wider, so if you want to engrave a 300mm wide object, it will require way more power than if you are trying to do a 150mm object.
Here is the technical explanation: Light follows the inverse square law, so as area doubles the intensity of light is a function of the square root of that. Not important that you know the math, but this photo should clear it up.