It's good for fire, lens-flares, water, and in some cases, acidic rain. I use it for light beams, too. Basically, it's like color-dodge (the layer type) in Photoshop, in my opinion.

Noel R. Cower Portfolio Athlon 64 3000+, 1GB RAM, 400GB HDD, Radeon X800 XT Onboard Realtek 7.1 Audio, WinXP Home SP2, Slackware 10.0