There are a number of reasons you might need a white underbase. One of the biggest is dark garments. Underbasing your design with white can make sure that your colors don't get lost in the darkness of the shirt. Certain inks, like our Wilflex Epic Lava line of plastisol inks are vibrant and opaque enough to be printed directly onto darker fabric without losing your vibrancy. An underbase can be used for several other purposes, such as to prevent the shirt from bleeding into the ink on garments like 50/50 blends.
The best way to think of underbasing is like you're giving your colors a stage to stand up on.