If you're mainly looking to store weapons, you can't get more efficient than a station full of Vipers and Vultures.
In my case, the module storage slowly fills up because I enjoy tinkering, and the workshop floor gradually accumulates modules I'm "not using right now" but don't care to sell because they're well-engineered or annoying to find. Or just simple impatience. Like, "oh look that's a tasty mission that I could do in one run if I just swap this HRP for an extra cargo rack, I'll buy it here rather than wait for one to transfer from the carrier" and after returning to base I transfer the HRP back home but 50/50 odds I forget to sell the cargo rack back.
Although FWIW I don't really use storage ships, unless you count a building a FGS or T10 to ferry weapons over to an engineer base en masse. When the module storage fills up that's the signal to do some spring cleaning, or possibly experiment with building an entire new ship from the scraps.