Apparently, coins are spent to perform Power-Up Fusions, and I was told that that is their only purpose. Is this true?

1 Answer

Usually, that is all that you can use coins for. However, it is currently possible to use coins to open special dungeons for an hour at a time. Go to the Shop tab to do this. (This appears to be a limited-time opportunity)

While the coin cost to open a dungeon is high, it's not that hard to get to with a high-level team. You get about 800K for finishing a run of the Legend-difficulty weekend dungeon.