Returning Player, a few questions

[chazus]

I pick up diablo every few seasons, and I'm actually pushing harder to get the Gaurdian reward this time around. I had a few questions that were nagging at me though.

1) Is there a purpose to having so many gems? I usually use maybe 10 on all my junk, but have hundreds piling up. What do I do with them?
2) Someone mentioned that ALL materials are useful now and to pick up everything these days to salvage. Howso?
3) Is there any legitimate purpose to crafting plans, outside of like.. Cains set for alts?

[Biomega]

1) Imperial-quality gems are used to reroll necks and rings, so you will need a substantial quantity of those for hard-to-get rolls like CHD. You also need top-quality gems to augment ancient gear, which is more of a high-end customization but is required at least once for the last part of the Season Journey.
2) A lot of good gear (primarily weapons) comes from upgrading rares in the Cube, which needs every material - white, blue, yellow, and Death's Breath. Also, you can convert between white/blue/yellow materials if you run out, which most people do frequently if they're upgrading and rerolling a lot. Crafting materials are one of the bigger bottlenecks.
3) Reaper's Wraps and Hellfire Amulet are used by a few builds this Season, because of the LoN buff. Also, the Sage's set is very useful now since it doubles DB drops (rather than just +1 as it used to). It's not that feasible for every class, but some can very easily incorporate it into a good farming setup and acquire massive amounts of DBs that way - which you can then use to upgrade rares in the Cube, giving you those big nasty ancient weapons faster. By and large, though, the crafting plans aren't super relevant that's true.

[redfinale]

1) Convert lower gems to higher gems. Can keep extra gems handy so that you aren't constantly removing gems from gear every time you decide to change sets, or to have enough for multiple characters (after all, in armor rubies, emeralds, and topazes give primary stat and at least two class use each). Also, gems have no level reqs, so putting high level stuff on lowbies if/when you wanna do a different class

2) Materials are not only put in their own inventory now to save space, but you need them for kenai's cube. the cube plays a big part in bettering your character for end game and not having enough of a mat sucks.

3) In most cases, not really. You could make a lvl 70 legendary, reroll a socket on to it, and then use a gem of ease to remove the level req and then give it to a lowbie, but it's far cheaper to just make a lvl 70 yellow and do the same thing, and other than maybe that odd legendary for some random build, it's more for "Teach So and so X amount of recipes" criteria for achieves and/or season progress. JC plans are useful however as it's the only way to not only upgrade gems past a certain point, but to make the hellfire pieces. The hellfire amulet is actually needed in some builds because of the freebie passive it can potentially give.

[Pattiecakes]

Kanais Cube is your resource and gem dump. It allows you to "target" gear and to augment what you have.
https://us.diablo3.com/en/game/guide/items/kanais-cube

[rogoth]

to answer 1:

gems are used by combining them into higher tier gems at the jewelcrafter, offering you more power per gem rank, the left over gems are used at the mystic on 'rerolling' stats on jewellery items as mentioned above along with using the kanai's cube to augment 'ancient' gear with more primary stats by combining gems together with the gear and some other materials.

2:

all materials have some function in the game now whether it's with upgrading rare gear in the cube into legendaries, rerolling stats at the mystic, crafting gear, there's several ways to make use of all the stuff in the game for crafting purposes, if you want to be ultra min max then the only items you want to pick up are 'grey'/'white' items as they salvage for the most materials per item then use the cube to convert them into dust or crystals as you need.

3:
aside from a couple of plans the majority are there now as just transmog items and not used in anyway anymore sadly, as mentioned above the sage set is super useful for farming death's breath in order to fuel item upgrading in the cube, reapers wraps are a very good option for those builds that unload all their resources into pure dps while a support player goes around generating/gathering regen globes, and the hellfire plans for obvious reasons.

[Thelxi]

1. There are two gem-sinks once you get to mid/end game: augments and rolling/enchanting jewelry. Once you have well rolled ancient jewelry and finished augmenting most of your gear, then yes there is no more point to gems. You definitely don't have to go out of your way to pick these up if you can't be bothered anymore.

2. Yes all materials are useful now. White, blue and yellow mats are interchangeable at the cube, and you need them to reroll gear and to upgrade rares. Death breaths are used for the same thing. Forgotten souls are now the most valuable resource imo, and their purpose is to completely reroll legendaries to get specific ancients.

One note on mats is that S17 changed things up a bit and bounties now give shitloads of regular mats, and t16 gives you tons of DBs. Imo there is no reason to pick up non-legendary gear anymore since you get regular mats so much quicker just by doing bounties. If you must pickup regular gear, white gear is most valuable since it gives most mats per item (all regular mats are interchangeable).

3. Sage's set can be used effectively alongside some class sets like Tal Rasha, but for the most part even sage's set is quite redundant now that t16 spams you with DBs. There are niche uses for a few recipes, but for the most part just learn and forget about them.