In addition to the other replies (which I agree with) about the psychological aspects, another aspect historically is that avoiding round numbers meant that change had to be given when making a purchase.

I.e. if you bought something that cost $5 you could just hand the $5 to the cashier and they could just avoid ringing you up and pocket the money. In contrast if the object cost $4.99 or $4.95 say they would have to ring it up in the till so they could open it to provide change to the customer.