The original reason was to be able to maintain a large fleet stationed on non-core planets, which requires these planets to be continuously supplied with all resource types in the event that a fleet is parked there.
In which the simplest solution is a planetary (PDC) based hanger large enough to accommodate the fleet. Ships in hangers do not use resources of any kind. While you could argue this as an exploit you could also argue that the ships are simply kept in docking births either in orbit or on the surface depending on their size etc, to allow for any RP misgivings you have about it just force all ships to take a few hours or so of doing nothing when needed to replicate them getting all the crews aboard and the fleet into a position to take action.
I can see why you might want to have a set stockpile to maintain the fleet from an RP perspective but each time any ship changes any component or you add/subtract vessels or commanders you could find the maintenance requirements also change. Thus you get left with the issue of if it more or less micromanagement to use set stockpile orders or to just also a repeat round trip. Or even is the RP factor even worth all that extra work keeping the colony stocked with exact amounts that constantly change.
For the above paragraph reasons I tend to make heavy use of planetary hangers and generally have three types, first is a smaller type that could hold a few of my smaller military ships only and acts as either a patrol base or part of the forward outpost of outer colonies, second is a much larger hanger that can hold an entire fleet at one time and is generally reserved for homeworld and what I consider developed core worlds only. Finally I have a couple of huge hangers that are used to house groups of commercial craft that are currently not in use, here I keep things such as specific transport groups that are not being used to develop a colony, or ships like gate constructors and so on. Really it just serves as somewhere to keep my task group list down to a nice tidy small number rather than having to scroll through all the time.
If you have a suitable industrial output you can get these hangers up and running quite quickly, simply get them constructed wherever you have the both the minerals and greatest construction output as parts. Ensure you have a transport that can carry as many troops as is practical to build so you can move at least 10-20 construction brigades in one go (multiple ships may be required). Combine all this with a dedicated freighter group that is designed solely to be able to move both all the needed PDC parts and minerals for assembly in a single trip. You then essentially have a IKEA style task group that can setup a planetary hanger well in advance of the colony slowly developing. Through in something to carry a large amount of maintenance supplies too and you can have them arrive with spares for the fleet to use while waiting on the hanger to be assembled.
