Board Thread:Love Live! School Idol Festival Lounge/@comment-26529791-20150625003841/@comment-24187738-20150625023450

The first main use of coins (also known as G) are for training idols with other idols. In most cases you will not run out of them unless you pick up a few URs early on.

The second use is to activate different bonuses during Medley Festival events, such as Extra EXP or Extra Event Points. Most people will run out of G here after a lot of Medley Festivals as the cost is rather high. I advise using it on the first bonus during MedFes: it boosts the chances of getting better rewards, which can include scouting tickets. This is currently the only way of converting G into scouting chances, though it's dependent on your luck still.