Does a planter on terra-firma needs to have a bottom?

If the container is intended to be on bare ground, it will not necessarily need to have a base. Having a base adds substantially to the cost. A bottom is only beneficial if the container is on paving, or a deck, etc.
If your concern is moles or penetrating grass, put down some chicken-mesh or cartons first.

To illustrate, compare the prices for two equally sized planters, the ‘Jumbo, on Ground’ and ‘Bottomless Raised Bed, Square’.