There are quite a few reasons I can think of off the top of my head. First would be the one Zeta has already mentioned, but there are many more:
- Ascended items (both weapons and armour)
- Legendary weapons (all require level 400 in some crafting discipline)
- Craft for other alts (just because you're 80 and haven't maxed tailoring doesn't mean it can't be useful for a lower-level alt) - plus, getting more levels and thus skill points isn't necessarily a waste
- Craft for yourself (once you level up, it's handy to be able to craft yourself a set of 80 exotics with the stats you want)
- Craft for money (spidy is your friend)
- Unlock skins (for achievements)
- Do certain "treasure hunts" like Mawdrey, or get other high-end items like Light of Dwayna
- Use up all that bloodstone dust (which is account bound and can't be used until you're level 450 in one of the disciplines)
More specifically, since you mention artificing, you can combine agony infusions (which will become important when you start doing fractals). Check spidy on artificing for specific recipes that can help to make you money.
Also, a note on that crafted backpack, the exotic level 78 one will actually give you better stats than a level 80 one, but not better than an ascended one (in my case, Mawdrey, which, incidentally, required being level 500 in one and level 400 in four more crafting disciplines).
In short, if you're a beginner, I recommend you craft to level up your lower level alts, or (if you're already level 80) to support those alts. If you're a veteran, you'll want to progress to high-end items, many of which will require crafting skills.
Four character slots (you get five per account without buying additional ones) give you enough capacity to train up someone for each of the eight crafting disciplines without having to pay to switch (you can have two disciplines per character; if you learn more than that, only two can be "active" and you pay 10 copper per crafting level to re-activate it). I try to have disciplines that make sense (e.g. huntsman and leatherworker for my ranger), but it doesn't really matter except for storage convenience.