Tourniquets could be a very useful First Aid tool to deal with bleeds such as those in Excavation Site: Wetlands. Poultices can do the same for diseases.
The restriction on using them in combat is likely for PvP balance so that certain specs like Feral Druid or Shadow Priest don’t have some of their most impactful abilities (i.e. Rip, Devouring Plague, etc.) completely nullified by an instant First Aid item.
I would suggest allowing us to use the items in combat so they can be used in certain PvE encounters, but add a 3 second cast time so they are not as easy to use in PvP.