The shards in the places mentioned by McMc can get you quite a few pokemon (by buying all of these said pokemon you also receive an exp share), Blast Powder (Allows you to use a mined rock once more), various rare items, payment for move tutors, payment to participate in puzzles that can give you an HM, fossils, and pokemon like Timburr, and also other various functions that I'm sure will be implemented sometime in the future.