An interesting point that hasn't been made is the comparison between stacking MUs up and the old fashioned stock.
Fifty years ago, we had locos and coaching stock on any journey longer than about two hours. In those days, locos went to depots, but locos are short and interchangeable. That meant depots could be small, tucked into whatever corner there was lying empty, and tended to do light maintenance on whatever came through the door. Obviously locos had designated home depots, but they frequently turned up all over, and so anything pressing was dealt with before sending it home for proper work.
Coaching stock on the other hand didn't need much maintenance. It needed cleaning and watering, but that could be done at any station it ended up at. Carriage sidings existed for proper work, but carriages just tended to turn up whenever they needed maintenance; you don't need to take a whole rake out of service if the second coach has a wheel flat for example.
This collectively meant that not as much space was needed to stable whole rakes of stock, just the locos. Whole rakes ended up in whatever siding was free when they weren't being used, but most stuff just moved around the network until it broke.
Over time, the locos stopped being used and replaced with MUs. The loco depots were turned into MU depots, but you get fewer MUs in the same space. MUs are much more intensive, if one bit breaks you have to stable the whole thing until it's fixed. But most of those depots are still the same small size as when they were designed for locos, and so you need more places to stuff the trains until they are needed, or to wait for maintenance.
This, combined with there just being more train nowadays than there was fifty years ago, means that MUs just tend to be stabled wherever there is space to park them overnight. This could be sidings or platforms, and over time this has become the norm.
We haven't kept pace with more and more MUs that take up a lot of space, and it's cheaper to let it sit in a platform somewhere than send it back to the depot, so it stays there until morning. MUs don't need fueling every day, and watering is easy, and they don't need as much tlc as locos did, so it works well enough.
