A majority of Engineering recipes require smelted bars acquired through Mining. Unlike items crafted through other professions such as Blacksmithing and Tailoring, many items crafted with Engineering are only able to be used by players with adequate Engineering skill. Engineering is one of the most popular professions in Classic because it is widely regarded as the best in many PvP and PvE dungeon/raid situations, as well as for twinks. It is also the only profession in the game to offer companion pets.
Unlike other professions, Engineering is the only profession that improves your character's prowess in combat. Let's take a look at what you'll be able to make:
Explosives: Items like Goblin Sapper Charge and Dense Dynamite, and Thorium Grenade bring your character great benefit to area of effect damage and an AoE stun.
Battle Pets: Raiders will want to get their hands on Arcanite Dragonling and Gnomish Battle Chicken to bring additional benefit to the raid group.
PVP Trinkets: Among the many devices tailored toward PVP you will find Hyper-Radiant Flame Reflector, Gyrofreeze Ice Reflector, and Ultra-Flash Shadow Reflector, these items allow you to reflect powerful spells back at the players who cast them.
Additional Hearthstones: While not exactly a Hearthstone, you can build either Dimensional Ripper - Everlook or Ultrasafe Transporter: Gadgetzan to give you another way to transport yourself quickly around an otherwise large world.
Gear: Items such as Force Reactive Disk provide unique and powerful effects that you won't be able to find from traditional sources of loot, this shield is used by tanks even in Naxxramas for AoE Threat.
Helpful Gadgets: Engineering also provides other devices that will help your guild out, you might be familiar with Field Repair Bot 74A which will allow you to repair your items without returning to town.
Engineering pairs well with Mining as lots of ore is needed for Engineering Schematics. However, if you are rolling Engineering, you are likely already thinking about min-maxing your character, and may choose another profession with optimal BoP perks (ie a Mage with Tailoring) if you have enough gold or an alt with Mining.
Here you can buy Classic Engeneering & Mining Leveling for EU or US Servers.
Also you'll get guide for completing Classic Engeneering & Mining leveling