Im level 21 nord Sneaky/assassin type, and haven't invested any perks into smithing, please give me some reasons why I should.

Because you kill things quicker and have better everything.

Well, there are a lot of reasons to invest time, money, and perk points into the smithing tree. The main of which is obviously the top-tier gear that you can forge at the higher levels.

Basically, you can create your own gear that is much better than anything you can find in the world of Skyrim. If you combine it with enchanting, you can make god-tier weapons and armor. The main thing with smithing is self-control. Smithing can get very overpowered very quickly. Just make sure you stop when your gear is acceptable.

you can get to level 100 smithing in a couple hours real time. It gives you the option to smith any kind of weapon/armor you'd like, and improve it to the maximum. It makes you harder to kill, and makes it easier for you to dish out damage. Not to mention everything you make and want to sell goes for more $$ helping you earn more gold in less time.

It's a no brainer really, unless you're role playing, no reason why you should pass up on this.

Well on all my plathroughs I never really mess with it at all..guess I'm missing out

Smithing can improve (most) Weapons and Armor.
Smithing can be used to gain experience needed to get your character from one level to another to gain a perk for use in another skill. I use Alteration on most of my characters, and at a Alteration skill level of fifty an unarmored Mage will need three perks, but I can use Smithing to gain the experience needed to gain one more level and gain a second or third perk to spend.
If you are at level 21 and have not used Smithing, then I do not need to warn you about spamming to get it as high as possible as quickly as possible.

The thing is to get your equipment to Legendary status. Taking the Steel and Arcane perks is enough, unless you want to craft your own gear. Using a combination of potions and enchanted apparel will help you get to Legendary. Getting Smithing to 100 alone is not that powerful. It's when you take your equipment beyond the Legendary status, that causes an imbalance in the game. I don't think the game was initially intended for that to happen in the first place.