So your attacks don't get dodged or parried and when that happens you can maintain your defensive abilities at close to 100% uptime. Blizzard wants active mitigation not passive to be the tanking model.
I dont know if anyone can break down to me why we need expertise at 15% and why we dont just soft cap expertise.
Because the soft cap is irrelevant for tanking now. Expertise's effect is exactly the same for us between 0-15% (whereas before it negated dodge and parry at the same time, so it lost value once dodge was fully negated). In order to maximize Holy Power generation by removing Crusader Strike's chance to be parried, we need the full 15%.
The reason for both the stats is just Holy Power generation
If a Crusader Strike is avoided it doesn't generate HP, then if you have haste you reduce the CD of CS and Judgement (along with many others) so you vastly increase the rate at which you gain Shield of the Righteousness
Now to get Hit/Expertise capped to you guys suggest in gem and forge for it?
Gem and reforge into it. Most of my gems are 320 expertise, if not all of them.
7.5% makes sure you can't be dodged
15% makes sure you can't be parried
7.5% Hit makes sure you can't be missed.
It all works hand in hand, if you miss - you don't get your damage reduction.
If you stack haste, you can make sure your SOTR buff falls off and with that, you can have near-enough a constant damage reduction buff up.
It also increases your damage dealt, decreases your damage taken and increases your healing overall.