Cause how the hell else are you going to level up your Cryptarch...
Also the more you loot, the more you dismantle, more you dismantle, the more materials you have
Go try to get 356 Spinmetals, took me 4.5 hours to do so
Now I can get 4 more, and trade them in 9 times either to the Vanguard or Crucible quartermaster and level up them quickly
Earn stuff along the way, or keep stockpiling, for the inevitable, Rare+ gear/weapons that require said materials...
You can get to 20 quickly, good luck getting the mats you need to upgrade
I can go and buy out the Green Engrams at the Cryptarch, repeat the process, level him up (very slowly due to nerf), then dismantle the trash I don't need and earn materials in the process
So my 18,000 Glimmer, just got me materials from weapons/gears, also leveled up my Cryptarch who will now better decipher those Engrams once he ranks/levels up =D