In any puzzle-like game the hints or helpful bonuses must doled out on a timer or players are likely to abuse the system. In mobile games, the hints are usually placed behind 30 second advertisements, using our natural hatred of advertising as a safeguard against abuse. It’s a natural fit.
Even a natural fit like this can be ruined by overzealousness, though. Too many ads punish a player who’s already stuck and frustrated. Yet as with everything in life, there’s a way to do it wrong and a way to do it right. It all comes down to presentation and pacing.