Humidity / Drip trays are important for healthy Bonsai Trees for the following reasons:
1 - Since all bonsai pots have holes in the bottom for drainage, the humidity / drip tray will protect the surface of whatever you are putting the tree on (windowsill, table, etc.)
2 - You should leave water in the tray all the time so that as the water in the tray evaporates, it will create a constant humid environment for the treeand help offset the loss of transpiration (the loss of moisture through the leaves.) Note: this WILL NOT provide necessary water to the tree's roots in this manner therefore you must adhere to the tree's watering schedule by watering down from the top of the soil.
3 - Instead of taking the tree to the sink to water it, you can simply water the tree in place. When watering with a watering can, pour water on top of the decorative moss (or pebbles) slowly until it penetrates the soil and the excess water comes out of the holes in the bottom of the pot. Then fill up the rest of the tray with water. This method is particularly convenient for watering larger trees.
Note - You will find the following abbreviations: OD - Outside Dimensions(space tray will take up); ID - Inside Dimensions (where pot would be placed)