This is what I understand about casual, midcore and hardcore. It might not be 100% true or applicable to FFXIV, but I think is the ideal.
Casual: Storymode, simple content that you can complete leisurely, at your own pace and in a short time, without even knowing whats going on. You can't do wrong. People identified as casual are usually parents or people with a busy real life, leaving them with limited playtime to do anything "serious" (like a 2 hours raid). Content like Titan HM/EX, Ramuh EX, Bahamut coil tuns 5, 9, 12 and 13 and even Alexander normal are WAY too much for them. For whatever reason, expending hours studying that content and trying to beat it is just not fun for them.
Hardcore: Raids, Savage mode, maybe EX primals. Content that requires a well organized group, voice chat and the best gear in order to successfully and repeatedly beat it. Everyone has to study the mechanics and be efficient in their role. One "lazy/clumsy" member can ruin the "hard work" of everyone in the party. People labeled as hardcore tend to care a LOT about their gaming experience. They strive to be "server first". Must have the best available gear, be able to say that they "beat it" -- whatever "it" is --, they know everything about the game, about their class/job/role, and of course, can dedicate hours and hours to the game. Not necessarily because they have 'no life', but because they want and can.
Midcore: Something between casual and hardcore. They can do raids, savage, ex primals and stuff like a hardcore, but they aren't as dedicated. They still tend to progress the game at their own pace, while hardcores are max level and all geared up, midcores are still a several levels behind and havent even start to gear up (meanwhile casuals are still just taking in the scenery and trying to decide if they are interested in the new content yet, or not.) Midcores might have been hardcores in the past, but for some reason or another, they cannot longer dedicate that much of their time and energy to games as they used to. They do enjoy hard-to-beat content, but they are able to take things slowly, and even enjoy old content with the casuals.
Anyways, that's how I would define those labels. I don't think they are stupid or unnecessary, I think they are just different play styles, very much in the same way we have different games preferences. Me for example, I consider myself midcore: I enjoy hard content, I study it, I can perform my role well and I'm able to carry my own weight and others. But I can't be bothered trying to be "server first" on whatever and not gonna bother leveling every job just so I can have all the crossclass abilities. "I will get there, when I get there... and will do it well".