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I opened up the wall to relocate my washer's water feeds and discovered an interesting double pipe arrangement. For each of the cold and hot feeds, there is an extra arm/appendix about 3feet long that dead-ends into an end cap. What's that for?

I'm guessing it's some sort of homemade water hammer arrestor? If so, why/how does it work (does it matter if it points up or down, or how far from the spout it is, etc.)?

double pipe

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Seems to be an arrestor.

They work by being pointed up so there is an air pocket at the top. the air pocket creates a cushion for the water when taps/faucets are turned off.

For the size you have, guess they had a couple of three foot lengths left over and did not want to cut the pipe again and had the room.

It might also have been added to add future plumbing to and seem like a good place.

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    Larger ones take longer to become useless due to the air dissolving into the water, increasing the time before you need to drain the plumbing to refill them with air. The "mechanical" sort try to solve that compactly with a piston separating water and air, but then become subject to the piston seal wearing out. These require the occasional system drain, but Keep It Simple, Sam. No moving parts, nothing to wear out.
    – Ecnerwal
    Jan 14 at 14:33
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    @Ecnerwal Guessing 'Sam' is the PC version to kiss
    – crip659
    Jan 14 at 15:29
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    Many washing machines use solenoid valves which slam open and shut causing huge vibrations. That is why the arrestors are on those valves.
    – Steve Wellens
    Jan 14 at 16:21
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    thanks. I didn't know I was supposed to periodically drain that to let bubbles in! I guess you do it when you hear knocking. I was going to replace the spouts (they stick out of the wall) with a recessed washer box which comes with its own arrestor, and now debating whether I should keep the current pipe arrangement or not (I'm moving to PEX-B). Also, edits are welcome on making the post title's more useful in the future -- I wasn't sure how to ask this question, and it might be hard to find as it is.
    – init_js
    Jan 14 at 20:40
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    @init_js Copper pipes last a long time. Would only change them if they were damaged or if I was adding more plumbing would I think to use Pex-B. Pex-B is good, but not good enough to change out good copper pipes just because.
    – crip659
    Jan 14 at 20:47

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