This helps to make the surface of the speakers smooth so sound does not bounce off any rough bits. The slate speakers do sound different from usual domestic speakers. So the lack of these sounds can be quite off-putting at first, but the reality is that these sounds should not be there in the first place. Wood vs. Slate Wood is an uneven, unreliable material which can change sound from speaker to speaker. Slate is a much more uniform material and is therefor more reliable.
Shape is all important.
Sound likes to run along surfaces, so sharp edges on wooden cabinets interrupt this flow. Slate has much smoother edges. The small size of the speakers belies their sound. These are the best you can get. Alan Parsons' Verdict 'Lovely sound but would probably be preferred by the classical listeners rather than people who like to Rock 'n Roll.