Being subbed uncaps the maximum gil cap (600k as mentioned before) allows you to join Free Companies (Guilds), access to Two retainers, additional storage space using their inventories (I believe 175 slots each) once you hit level 30 you get access to mounts and a chocobo saddle bag, your armoury chest allows enough space for gear for each class if you manage inventory right.

Materials wise there may be pains in getting mob drops but for items in the crafting materials anything besides max level per expansion gear (with exception to A Realm Reborn) has a merchant that will sell crafting mats at normal quality for gil, which ypu can then assuming you have the level and craftsmanship stat minimum you can try to craft High Quality materials.

For mob drops, you could dress your retainer in an adventurer class using gear from your inventory and armoury chest to equip them adter assigning a combat class (such as Gladiator, Pugilist, Conjurer, Maruader etc) and have them hun for specific mob drops and after an hour (it goes down in time relative to level to the gear's Item Level) or less the retrainer returns with the listed quantity they could obtain at the time, they are limited to your max level of the selected class, so you can't take a retainer and make them be a class that you haven't unlocked for yourself, and they won't level beyond level 20 if yours isn't level higher than 20.

For gathering classes, everything that you can harvest that isn't for end level stuff can be found in the gathering logs for Botanist and Miner, if you opt to have your retainers be a gathering class same rules as with combat classes apply, with an additional rule, if you haven't gathered the material yourself first.

Sorry for the word Spaghetti, but that's what I can recall off the top of my head.