Crafting and gathering are optional, in fact you don't really need to get them if you already have friends in game who can do that. However, the benefit of having them for yourself is profit first thing, there are a lot of stuff that you can gather and make all day for gils. If you are already on your relic questline, you will need crafted items to advance at one point of that quest so if you can make them you save alot of money for yourself and won't have to bother someone else to do it. If you like a different kind of challenge beside the usual raid/dungeons, you can go for it.