My opinion only, not by any means a settled view, but I've always thought that playing a lot of chess helps you to be good at chess, playing a lot of sudoku helps you to be good at sudoku, doing crosswords a lot makes you good at doing crosswords, doing a lot of IQ tests makes you good at doing IQ tests and using a metronome makes you good at keeping up with a metronome. Evidence that these things improve mental skills more generally, or stave off dementia, etc., is notoriously hard to obtain. That is not to say that they are not good things, but there isn't enough to go on to say that they should be prescribed as mainstream educational tools. The null hypothesis has yet to be defeated.