Hive tools get sticky. They come into contact with propolis and honey. If they build up too much sticky “stuff” it gets harder to use the tool effectively.
That's one of the reasons to have a 5 gallon pail of water at hand during inspections. This allows the apiarist to clean off the muck every so often
Hive tools also come into contact with things not seen directly or until too late. Things like mold, mildew, contaminated wax, dead bees, larvae, etc…
Bacteria, viri, fungi and other carriers of disease and illness can be transferred from one hive to another via the hive tool.
Having a bottle of isopropyl alcohol to douse tools with between inspections can help reduce transference a lot. A simple wash at minimum is good. A wash followed by a go over with a propane torch can really reduce the potential for transference.
Washing things like hats and veils, gloves (if you use them, usually not permitted in my apiaries) and other wearables is encouraged.
When bees sting gear, the alarm pheromone can build up and invite bees into aggressive defensiveness simply by the door of the unwashed gear. Reduce unnecessarily distressing bee colonies by keeping gear cleaned between outings.
If there is an immediate need in between hives or apiaries to knock down pheromone scent on gear, use smoker to cover scent. Also, Febreze unscented spray can help greatly to eliminate scent from gear in between apiaries. Make sure to give gear time to dry out as people tend to over-spray and get gear set or damp.
People sweat wearing PPE gear. Bees aren't often find of the smell of sweat. Washing between outings keeps gear fresh and clean to prevent unnecessary distress to colonies and apiarists.
Even during routine inspection, we might observe a buildup of propolis, burr or bridge comb, and general messiness frequently resulting from bee space violations inside the hives.
The primary reason for cleaning equipment like this is to facilitate future inspections and activities with little to no mess to slow down or prevent those inspections and activities.
The tops and bottoms of frames are easy to see this messiness and should be scraped off as you go along. Collect discarded material that has been scraped into plastic bags or another portable container so as not to leave detritus on the apiary grounds to attract predators and pests.
The inner cover and hive box sides also need to be scraped from time to time.