It is five times easier to sell to existing customers than it is to attract new business, so it makes sense for resellers to sell their customers as many complimentary products as they can. This also stops competitors gaining a foothold in their customer base.
One person to call
For customers, having a single-source supplier makes sense, as they only have one person to call to report an issue. Often when resellers only manage the hosted voice solution and not the underlying internet connectivity, it is more difficult to analyse the fault and fix it quickly. Similarly, you can only guarantee an improved SLA if you buy the primary circuit and 4G back-up from the same supplier. Customers often feel fobbed-off by suppliers when they ring to report a fault with their telephony, only to be told to re-dial their internet supplier, as there is no known fault their end.
As a business if you feel valued and rewarded for buying several products from one single supplier, you are less likely to churn away, especially if you have a good deal and experience great service.
Having one supplier, businesses only have to worry about one bill and can easily scale up or scale down if they are on 30 day contract, per user, per month terms.