The range of these weapons means that the IS could bombard Iraqi or Syrian cities from as far away as 20 miles. This means that they needn't be a close-quarters combat military force. This further goes along with the analysis that they are acting more like an army, as they have now been described as an"Islamic army" by US officials. These weapons have most definitely enhanced their military efficiency and effectiveness against Kurdish and Iraqi forces since they took the powerful weapons back in June.
It goes without saying that these howitzers are just part of the myriad of American military equipment supplied to Iraq that the IS has taken from the Iraqi military. These weapons are now being used on the very same people the weapons were originally intended for; as well as being used Kurdish forces. Interestingly enough, the Kurds have been taking some American-supplied weapons from the IS. Personally, I think it would just be easier to supply the Kurds directly, rather than have them get their equipment by us sending weapons to Iraq, then have the weapons taken by IS, only to be taken again by the Kurds.