So what are you supposed to do with your combat classes?

[Reinha]

Titles, minions, mounts, hair styles, music rolls, accessories, bardings, triple triad cards, glamour gear and housing items. You will need your combat jobs to get those. WoW also relies on cosmetic collections to help you decide what to do.

[Jeeqbit]

Roulettes, MSQ, sidequests including beast tribes, extremes, eureka, bozja, deep dungeons, pvp, hunts and hunt spawning, maps, collecting things like mounts, minions, cards or relics.

It doesn't all have to be with your combat classes, because there is the gold saucer with jump puzzles, chocobo racing, lord of verminion (minion battles), triple triad, doman mahjong (most of these games can be done as pvp and have tournaments). There is also crafting and gathering with features such as fishing, ocean fishing, a gathering area called diadem, the firmament, custom delivery stories and their own beast tribe quests.

[Cabalabob]

Personally I try to run different content on different jobs so that I have some variety in my gameplay. For example I tank savage, heal ex trials, go ranged for whatever relic content gets released, melee for roulettes and caster for 24 man raids (all assuming I’ve max levelled all jobs and I’m not after job specific loot).

Also makes me a more well rounded player so if I’m needed to flex in something I have experience with all roles.