I would guess a lot of it is to deal with method claims.
Where an apparatus claim covers the device itself, a method claim covers carrying out the steps outlined in the method. In quite a few cases, that could be summarized pretty simply: the patented method is using the product as intended, for its intended purpose.
In other cases, however, it's going to be narrower than that though. Especially with "convergence" devices that can/do include quite a few features that are only minimally related, it's entirely possible that a particular patent might apply only if the device is used in a particular way under specific conditions.
The mere existence of the product doesn't mean anybody will necessarily use it in the way specified in the patent, so it's often nearly impossible for the vendor to way with any certainty whether or not you'll use the device in a way that would infringe.