Well a number of reasons.
What if you lost your job?
What if something else came up which meant diverting money to?
Why pay 7.9% on the £30k when you could pay it off in 1 go?
If I had £30k in the bank I wouldn't buy a £30k car. I would buy one for less.
He would be paying 2.9% on the 30k, hardly much. He would also retain access to the funds on demand.