From the crunch side of things, the devils profit from getting a king's ransom of expensive gems to help summon devils into the material plane. Infernal Calling, Planar Binding, and probably in some cases, Planar Ally all make use of 1000gp gems. Sure the devils would typically be bound by some terms when summoned with those spells, but devils are adept at cooperation and weaseling in technically legal acts into their tasks, letting them make subtle but big moves when they can get many devils on a task.

Sure someone could tell a pair of devils not to burn down a temple, but if one of them likes to smoke and another really likes high proof booze, 'accidents' could happen.

If you want devils to get involved subtly, a sorcerer* bad guy with subtle spell is an easy way to subtly summon devils. Yeah, a royal is wearing a necklace with a 999gp ruby set in it. Why shouldn't they? Also a long term Planar Binding or Planar Ally could allow for a devil to communicate with its summoner through subtle means, like the flags flying over the castle on a certain day, the smell of different things getting burnt somewhere, or something like that. A devil could also straight up lie about its identity. If the bad guy prays for an angel to save them and an Erinyes swoops down to smite some fools from longbow range, even people with Detect Evil might not know the difference.

*Or someone who has Metamagic Adept and some way to cast Infernal Calling. Spell lists can be tricky to keep straight. IIRC an Eloquence Bard who uses a magical secret to learn Infernal Calling is the best way to use the spell and it's kind of a hassle any other way.