Hello, I am at rank 37 atm, stealth 8, armor 12, power 12... (where 12 is max)

And I don't understand - like could someone please explain to me - what's the point of getting new XP points ? Like why do I have X unlocks in stealth, power, armor when I can only use one of each category at a time ? And all of those I want to use I already use and they are all upgraded to max level 3 ? What am I missing ? Similarly - can someone explain to me - is there any difference between stealth lvl 8 vs. lvl 12 (maxed) ? (12 uses less power, more "invisible" etc. ?)

Also what is the point of getting weapon unlocks if I can have only 1-2 primary weapons and SCAR is clearly superior to any other weapon in the game ? What is the point of having so much more unlocks on SCAR weapon than you can use at one time simultaneously ?

Granted - if I earn enough XP I'll progress to next rank 38 and so on till 50 - but what will that change ? If I am already using what I think is the best combination of weapon / armor / stealth / power modules ? Will rank 50 give me any advantage over rank 37 other than on paper ?

the max 12 lvl means you unlocked stealth/armor/power perks