Concrete under timberfloor in laundry - why?

This is my first post and I hope its not a stupid question. In the corner of my laundry (1950's renovation) is a 1m x 1m timber board that was screwed on top of the timber floorboards. I am doing a renovation and removed the timber to find that there was a block of concrete approx 1m x 1m under the timber board. I need to remove approx 30mm of this concrete slab to be able lay new floorboards. Does anyone know why the concrete could be here? Were old cement troughs cemented 30cm below the floorboards?


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