Rock Smash. It breaks rocks, sometimes it avoids a trainer battle but what is the point of it, rather than avoiding trainer battles?
I believe the only games that require Rock Smash to progress after a certain point are R/S/E, D/P/Pt, OR/AS, and BD/SP. Note that "progression" in this case is just referring to requirements to reach and defeat the Elite Four and Champion. I believe there are some postgame bits in Firered and Leafgreen, for example, that Rock Smash is required to fully traverse.
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