Originally Posted by Strangelove77
And leveling up your account level does what exactly? You have better stats?
No better stats no.
Its a slow progression meta level that has an effect on a lot of game systems, especially tings like daily caps. As you gain Mastery Ranks you gain the ability to craft new weapons (that are Mastery locked), raise your daily caps in terms of bonus Syndicate Reputation gains, void traces, and trading limits, and more.
Every two levels you gain a saved loadout slot (Warframe+Weapons+Gear+Companion), and each level of Mastery is basically a free (advance on a) level of experience on weapons and frames and companions. This is a little tricky though, so consider that at starting mastery with an unranked weapon or frame you have zero mod capacity to work with in terms of slotting mods. If you're rank 16 Mastery, your unranked weapons and frames can slot mods as if they were level 16. The side effect of this is you don't gain any mod capacity until you rank the item from zero up to beyond level 16 at which point you start to gain more capacity to slot more mods as usual.
The end result of this is that the longer you've played the the more power you can squeeze out of your low level gear from mods, and thus the quicker you can level them up.
These days whenever I start an unranked loadout, I usually head right into level 15-20 missions because of Mastery rank bonuses.