since I'm only level 3 with 24 destruction at the moment what is the purpose of dual casting I already have sparks in one hand and flare in the other and have been casting them at the same time / seperately ...

If you have Flames in each hand (or Sparks or Frostbite, etc.) then it puts them together when you cast it to make a more powerful version of said spell.

Go ahead, equip Sparks in each hand and try it out. You'll feel like Emperor Palpatiine in no time!

been also using sparks in both hands mostly (love lightning wish i could afford more spells ...)but i dont even have dual casting perk so why get it ?

I was asking myself the same question. :/

Dual casting is something like this:
2.2 times the damage of a normal spell, casted with one hand.
2.8 times the magicka of a normal spell, casted with one hand.

Example:
Casting sparks with one hand does 10 damage per second, and costs 10 magicka per second.
Casting with 2 hands without the perk will cause 20 damage per second and costs 20 magicka per second.
Casting with 2 hands with the perk will cause 22 damage per second, and costs 28 magicka per second.

Dual casting also allows for the stun-perk to be unlocked. This is a very powerful perk, since you stun almost anything. (You stun everything when you take the corresponding damage increase perks)

Because it effectively almost triples the effect. In each hand, you have one copy of the spell, which is double the effect. With the Dual Casting Perk, the effect is heightened enough that the effect is almost triple that of when you one-hand it.

This is the entire point of dual casting: stagger from the impact perk. It's very powerful but it does not work for the novice spells so if that's what you're using, don't bother. Once you get fire bolt, try it out, you'll be surprised at the difference.

I dont think its worth it until you get impact. Costs too much mana. Just keep sparks in one hand and flames in the other until then.