Runes and runewords provide a LOT of different, good things to the game.
End game chase items, some very very rare so something to look forward to dropping.
A form of trading currency, since some were so rare and they'd be taken out of the economy by being used.
Could have potentially been a form of mystery, with a new runeword being something to hunt for.
Had another use besides being used as a whole runeword - gave player options for adding small stat increases in their gear.
I'm personally pretty disappointed in how their systems are being developed so far. I think they got the art right, and I'm sure combat feels good, but that's about it. I don't want to get all hyperbolic, but I'm REALLY insulted they are choosing to call this system 'Runewords'. A trigger and effect system? Sure. But they chose to cannibalize such a beloved part of D2, and something people have been begging for?