Quickcast is a mechanic of dota 2 that is sometimes(often, actually) misunderstood. Many people view it as difficult, weird, or have never heard of it. and as everyone knows, you can't learn something new if you don't know what it is. Simply put, Quickcast allows you to "flurry" or cast many spells within a small time window. This is very useful for all heroes, but especially ones that rely on flurrying to get their jobs done, such as
Tinker, 
Skywrath Mage and others.The way quickcast works is that, when a key bind for a skill or item is toggled, said skill will automatically be released at the cursor's current location. This means that, instead of hitting the key bind->moving the cursor to the target->clicking on the target/target area, you will toggle the key bind->and voila, the skill will be casted. This allows many spells to be casted quickly in succession.
Now for some pros and cons of quickcast....
PROS:
-allows for pinpoint targeting the moment of
-spells cast be cast faster in succession, this leads to better performance and reaction time
-useful for spells where judgement calls are often needed(
Fissure, for example)CONS:
-unless you have quickcast spells toggled to separate keys than regular spells, you will not be able to double-click to self cast anymore. You will have to move the cursor over your player.
-much easier to mis-click and accidentally use spells.
I personally am a huge fan of quickcast, ever since it was recommended to me by a fellow player, and also by TheGooGaming in one of his guides. I personally think that, while it doesn't make the game of dota 2 easier, it makes reaction based spells, and really just spells in general, much faster, which leads to a better win rate. In my humble opinion, quickcast can mean things like landing that
Fissure fast enough to block the enemy 
Faceless Void, which in turn lead to a tier 2 and a lead for your team. It's seriously that good.
I am seriously thinking of starting to use quick cast.