They are used because in an emergency they are faster to get out of the holster, but are just as secure as hip holsters.
Sure, they're like
the old West quick-draw McGraw holsters. I'm sorry. LEOs manage to gun down people every day with regular waist holsters. How often has an Amtrak train required... Oh, that's right, there was that
Dallas case in 2011 where LEOs gunned down some miscreant, as well as shooting one of their fellow officers and a bystander.
You know, maybe quick draw isn't the important parameter here.
I think that it's dress-up, just like LEOs wearing paramilitary outfits instead of proper uniforms. But hey, I'm old school. I'm mystified why soldiers who work in offices wear BDUs.
Traditional police uniforms are more ceremonial. They must be dry cleaned and pressed and they are very expensive, most LEOs get an allowance anything over they must pay out of their pocket. The leather duty belt is shiny. The name tag, badges and emblems must be polished.
When part of a special unit, sometimes called; SWAT, ERT, ESU, and warrant service etc., there is a high likelihood of having to crawl through windows, ladders, rooftops, etc., as is expected of officers on SWAT call outs, serving search warrants, etc. BDU style uniforms serve the purpose much better than the old traditional police uniform. Some SWAT teams have two uniforms, traditional black and another color (i.e. OD green, camp, etc.). They're there to protect you and your family. Law enforcement serves an entirely separate purpose than the military. There is an equal need for both. I don't think one is trying to mimic the other.. All of these activities would damage or destroy the traditional uniform. The BDU style uniform can be machined washed and is much cheaper than the traditional uniform. It also has a more relaxed fit for the physical activities described above. As a cost saving, instead a police department designing their own custom colors, patterns and designs, its cheaper to just purchase military surplus.
Just as a note, every large department that I know has several uniforms for specialized units, e.g. weapons training in a close by department uses a bright yellow shirt over OD pants and a black baseball cap. That is so they came be seen and easily identified on the ranges. Also when wearing Class III body armor, you cannot wear a hip holster, so some sort of drop holster is required.
Requisite train subject, Amtrak PD has at least four different uniforms.