Clear rubber hose purpose for pressure washer

"what is the justification for not including them as a matter of course?"

• there are numerous choices when it comes to type and length of hose. Cheaper rubber hoses, more expensive stainless steel braided jacket rubber hoses, lengths from 4' to 12' long, etc.

"Can old fill hoses be reused? If not, why not?"

• They certainly can be reused. If you are confident enough that they are in good shape then feel free to use old hoses (you should replace the washers at the end connections).

The downside is that, unless you turn your water supply valves off every time you leave your house, an old hose that fails can easily cause catastrophic flooding of your home. You need to make your decision based on a risk assessment putting the cost of new high quality hoses against the chances of and cost of a hose failure.

I bought brand new supply valves with quick 1/4 turn ball-valve construction to facilitate easy shut-off, and the best quality braided stainless hoses; and I shut off the valves whenever I will be gone for more than a day, because I do not want to come home to a flooded house.

It's difficult to answer this question, without actually seeing the plumbing leading to each fixture. Reductions in pipe size, differences in pipe material, pipe length, number of elbows/tees, etc. These could all contribute to a reduced flow.

If you had a leak large enough to reduce the flow by half, you'd likely either notice it as water damage, or on your water bill. Reduced flow being the only symptom, a leak would be low on my list of things to check.

I'd likely start with the sillcock itself, to make sure it's operating properly. If the valve isn't opening all the way, or it's gummed up with junk, you'd definitely see a reduction in flow. Turn off the water to the fixture, and remove the sillcock.

NOTE: That's a frost-proof anti-siphon sillcock, so it actually connects to the supply plumbing somewhere inside the house. So you'll want to have a bucket under that joint, to catch any drippings when you remove the sillcock.

If everything checks out there, start working your way back along the supply. Look for pipe size reductions, material changes, stuck valves, or any other potential restrictions.