What is the purpose of the brown pads on this PCB?
It's an epoxy adhesive e.g Surface Mount Adhesives (SMA) for SMD components. It's typically used in surface mount Printed Circuit Board (PCB) assemblies to hold the passive, (and sometimes active), components on to the bottom side of the board during wave soldering. It can also be used to increase the thermal conductivity from the component to the PCB for better heat transfer.

If you look closely at the board you may notice that other SMD components on this same side of the board are glued in place with the epoxy. For example look at two other non-populated sites at J1 and J2. You can see some of the brown material there too:

 

The process being used to apply the adhesive could be using a screening procedure that was designed do the whole bottom of the assembly at once. And rather than having several screens only one is used despite the component population configuration.