Three quick reasons come to my mind.  The storage is offsite, it enable you to have multiple layers of backup, and the cost is really quite low.  As jimlynch pointed out, there is always that element of concern about your data being out there under the control of someone else, and you being forced to rely on the security measured decided on by a cloud provider, whatever they might be.  I would be willing to wager that most of those cloud providers, at least the large, well known providers like Amazon, have a much greater focus on security that most small businesses ever will.  I do understand the inclination to purchase your own storage and not face ongoing expenses, but you also have to take into consideration the lifespan of those big HDDs you mentioned, and the time value of money when you look at ongoing smaller expenses vs. larger up front costs.