Fractional banking acts as a multiplier on the money available for loan takers.
Actually, this is subsequently to having taken loans that the money multiplier increases from 1. Fractional banking potentially increases the "money supply", it does not do so performatively. Incidentally this is even why, western economists are currently discussing the possibility of fighting the next downturn with helicopter money.
If I have borrowed, say, a bike, and that I do not plan to use it during the next week, why preclude someone else from borrowing it from me? And the risk that I change my mind and need my bike during the next week is not unmanageable. Especially that the new borrower may have good opportunities to increase to collective wealth.
What would happen to the economy if fractional banking was gradually abolished?
Resources would be suboptimally employed and we would probably enter a long period of recession. Especially that the world currently "needs" inflation and that credit is the main conventional inflation-engine. That being said, at some point, the entire economic system would likely synchronize with this new credit pace, finally leading the world into a new economic regime, slower, with less volatility and with longer business cycles.