Presumably Drain Punch and Substitute are there to aid with longevity, if it is able to switch into a decent position. Poison Heal also does this. It could be quite efficient if it were brought in later on or substituted in. Generally speaking, the overall point seems fairly clear in a way - it begins with Poison Heal and goes on from there to other moves which either take advantage of this, like Substitute, or complement it like Drain Punch. Rock Slide, as noted, might be in part due to flying types and so on. Generally speaking, Drain Punch is likely assuming that it's used against opponents who are Rock type or otherwise weak to fighting attacks, where it will be more effective, and if it gives passive damage or they switch out then it does have a useful healing effect. Obviously, it isn't mostly designed for giving significant neutral damage, but it does help keep certain opposing types weak over the course of a battle, and has a decent chance of surviving common general attacks like Earthquake. Since Breloom is in part used due to its being a useful Pokémon for type coverage and ruling out several attacks against it as well as being effective against several prominent types, that people try to keep it going over the course of a battle needn't be that surprising.
Whether or not it's maladjusted to Breloom itself, the moveset itself does have some sense, it just might not be applied to the right Pokémon.
