I suspect that there are, at root, three reasons for stats.
First, they offer a baseline mechanism for certain things not covered by skills. . .unless you really want to go all out with skills like:
Encumbrance
Initiative
DB
And more in that vein.
Second, there are races, which gain a lot of their variation in stats.
Third, it's the primary layer of skill/task similarity. . .guns are like acrobatics are like bows are like dancing is like fencing is like the hurdles is like juggling. . .all reflected in agility.
I'll horribly miss-quote Defendi, but hopefully catch the gist with:
"Rolemaster is great for it's splendid, almost arcane structure of complex interactions and interrelations."
So it's not just an archer with 10 ranks
it's not just an archer with 10 ranks who's a fighter for +20
it's not just an archer with 10 ranks who's a fighter with +20 and a strength +5
it's not just archer with 10 ranks who's a fighter with a +20, a strength +5 and an agility +7 applied twice.
It's not just an archer with 10 ranks whos a fighter +20, strength +5, Agility +7 applied twice and. . .he's a dwarf.
Toss some talents in, a quality bow, magic bonus, tango. . .
Each layer you add that has a different +/- axis, does not ADD a layer of detail / granularity / variation. . .it multiplies. . .
Stat, Skill Ranks and Professional bonus is expressed as X + Y + Z, but it generates variation as X * Y * Z across all possible skills/tasks.