The most obvious reason for utilizing a seed tray is to eliminate ground waste. For us humanoids, it’s the unsightly mess and sprouting weeds that are annoying. For birds though, it’s really a health issue. Wasted seed tends to mold, it gets dirty with birds’ droppings and it attracts other critters who can transmit disease. Many avian diseases are airborne and spread rapidly affecting much of the local bird population if the mess or dirty feeders are left unattended for long stretches of time. This is why maintaining clean bird feeders is so important.
Adding a seed tray will pretty much eliminate this problem. There are a bevy of models from which to choose, including pole-mounted and post mounted. Hanging trays are used by attaching them to the feeder itself.
The other advantage is that a seed tray will attract more species of birds. Those who may not actually perch or eat from the feeder will find the platform area with spilled seed very inviting. Which brings us to the spilled seed itself and the savings in money spent on that seed. So, it serves more than one purpose to add a seed tray to existing bird feeders, as it creates a clean feeding environment and attracts more feathered friends!