The depot is a
planned new structure which can be anchored like a container and act as a mobile base, with a ship maintenance array and a corporate hangar array. The full details are not yet known. In principle, it should be possible to build towns out of these.
To my knowledge, no direct advantage from anchoring depots close to each other has been announced, although a lot of ideas, like unlocking station services, shields etc once a certain number of depots is on grid/connected and so on have been circulating on the forums.
Trading between strangers in the middle of nothing is based on the container mechanics. In low-sec, a group of depots would be like having all the eggs in the same basket. It seems unlikely, that commerce or defence would do much to incentivize town formation.
If the anchoring mechanics does not change, then the depots can be within 5 km (3.1 miles) from each other. The two densest formations which can be built with this constraint are the face centered cubic (FCC) and hexagonal close packing (HCP) structures:
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If a chosen depot is buried deep in the interior of such structure, then it might be difficult to access the hangar and maintenance arrays due to bumping mechanics.
In some systems there are gates, landmarks and stations within a few ten thousands of kilometers apart. Creating celestial bridges between them has so far relied on corpses which last only for an hour, and on containers, which only last for 30 days. With depots, one could in principle connect the grids for an arbitrary length of time.
What other reasons there could be to anchor the depots close to each other, other than aesthetics, or pure RP?