What would be the reason for doing side by side replenishment like this instead of the regular underway replenishment? I imagine getting more practice in doing underway replenishment would be better. This way of replenishment at anchor seems to be commonly done by the
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Because the side by side replenishment isn't really just replenishment.
It's more of a "break day" combined with resupply of small boxed goods that can be transferred by just walking from one ship to another across gangplanks
Steel beach picnics in other navies are kind of similar in the sense that a given ship stops in the water and has some time to relax etc. I think this is slightly similar but less extravagant. They play basketball on the deck of the replenishment ship sometimes, from past memory.
The resupply or boxed goods is just something convenient to do at the same time as when the ships are tied up alongside each other.
Sure, they could probably do underway resupply or vertrep of those boxed goods as well (and I imagine they do), but those processes are fairly technical and time consuming. If they just want to incidentally resupply a few boxes of stuff, doing the whole song and dance of underway replenishment seems like a waste to me.
If it was during war, or if time was of the essence, then that would of course be very different.