A New Planter

I suspect it will be a barrier to enable water storage at the bottom, and/or drainage to prevent compost getting wholly waterlogged. I'd just put the compost on top and plant as usual.

Only insert I've seen in planters is to provide a reservoir at the bottom to extend the period between waterings, in which case there is usually a plastic pipe which attaches to the insert to allow the reservoir to be filled directly from just above the soil level.

If no pipe, nor means of attaching one, I guess it must be to help prevent waterlogging, as Yorkie has already suggested.