The main purpose of a collector tray is to collect liquid from bottom of the bed either for partial withdrawal or recirculation. It is similar to deck type distributor with no drip points but with gas risers. The volume of liquid is controlled by the height of the riser.
In these type of collector trays liquid is pushed into peripheral sump via vanes & troughs through which the draw off can be done. This offers high open area for gases and lesser pressure drop.