Long Reach makes Decidueye's contact making moves non-contact, which means they aren't affected by Rocky Helmet, Iron Barbs, Rough Skin and burn, paralysis, sleep, etc. from Static, Flame Body, Effect Spore (though he isn't affected by sleep), etc. And from some abilities like Mummy, Wandering Spirit, etc. Although it can be at least of some use, there are better abilities than that. Since Decidueye has only Overgrow apart from that, which is worse, Long Reach can be used.