I'd not use them, you run the risk of them inhibiting root growth and they'd block water which would possibly rot the wood. If it were me, I'd put the gravel at the bottom, to facilitate drainage, then just top up with soil.
yeah, it does seem like the concrete slabs are a bit unnecessary.
Thanks
@punkture started
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I'd not use them, you run the risk of them inhibiting root growth and they'd block water which would possibly rot the wood. If it were me, I'd put the gravel at the bottom, to facilitate drainage, then just top up with soil.