A quality pool table slate is key to a level playing surface and allows the pool balls to roll along the felt smoothly, without curving or taking sudden alarming turns!
In contrast, dents and cracks on old billiards slate become "wobble spots", places where balls do strange things. Slowly roll balls over a used table to look for indications of cracks or a poor level.
A one-piece pool table slate is the most expensive type, but three-piece makes for a superior table, slates assembled from several pre-fabricated sections, as one heavy slab of slate can sag beneath its own weight.
Never buy a billiards slate that cracked and was glued together again without care, unless the reassembly was done by an extremely skilled professional repairman.