Why use a humidity/drip tray? Several reasons come to mind.
Even though they are called humidity trays, most people like having a tray under their bonsai trees for one main purpose; to protect grandma's antique coffee table!! Since bonsai pots have holes in the bottom to prevent collection of excess water, this means that the water has to go somewhere and it is usually all over the surface that is under the pot, i.e., the table! A drip tray prevents this.
As a humidity tray, it delivers the necessary humidity to keep your bonsai healthy and bright. As the water collects in the tray it evaporates, thus rising in the form of vapor that is absorbed by the leaves. So if it is a dry climate you live in, a humidity tray is a must for the health of your trees. When used as a humidity tray it is suggested that you place a layer of gravel in the tray so that the gravel keeps the bonsai pot lifted above the surface of the water that is in the tray. This prevents the water from being "sucked" back up into the bonsai pot, which could help cause damage to the roots of the tree.
With a tray under your tree it makes it much easier to water that tree in place without the worry of water getting all over everything. Rather than having to water the tree in the sink, just water it as it sits on the table. But be careful that you don't use more water than the tray can handle with overflow, or you have just ruined grandma's table!!