What are these used for?
As above re the evolution items (also the relevant item shows up next to the pokemon's evolve button in your bag) and also yes re Machamp and Blissey. For what to keep and not, this site has a decent summary of all the good mons and their best moveset(s) if you search them in the bar at the top of the page as you catch each new one, although don't be afraid to keep something and try it out for yourself. Eevee day today and tomorrow offers you a really good chance to get a selection of really useful stuff of various types that will make you competitive. Be sure to check out how to get Espeon, it's the best evolution but they're all pretty good bar Umbreon.
Edit - keep plenty of Eevees for after community day so you don't get stuck with Last Resort as the charge move for all your mons - check out the Eevee "name trick" if you haven't already used it.
Certain species require such items to evolve.
Sun Stone: Gloom to Bellossom, Sunkern to Sunflora.
King's Rock: Poliwhirl to Politoed, Slowpoke to Slowking.
Metal Coat: Onix to Steelix, Scyther to Scizor
Dragon Scale: Seadra to Kingdra
Up Grade: Porygon to Porygon-2.
The best non-legendaries are:
1. Machamp (Counter + Dynamic Punch) as an essential gym attacker that also excels in many tier 4 raids.
2. Blissey (Zen Headbutt + Dazzling Gleam) as the best gym defender and thus your main coin source.
3. Tyranitar (Bite + Crunch) which excels in many raids, notably Alakazam, Gengar and Mewtwo.
4. Dragonite (Dragon Tail + Outrage) is a great generalist.
Hariyama (Counter + Dynamic Punch) is a slightly worse Machamp, while Salamence (Dragon Tail + Draco Meteor) is a slightly worse Dragonite.
Depending on your biome, Gyarados (Waterfall + Hydro Pump) and Exeggutor (Bullet Seed + Solar Beam) may be as easy to acquire as the Eeveelutions.
If you lack legendaries, save up Rare Candy (earned from raids) to power up a Raikou.