For some monarchs in the 19th century it was definitely an attempt to imitate Napoleon. The practice wasn't actually established or first popularised by Napoleon, however; Napoleon himself was influenced by Frederick the Great, who did the same (almost every painting and depiction of Frederick has him in military uniform), who was in turn influenced by his highly militaristic father Frederick William I, who seems to be the first prominent European monarch to have done this. Napoleon's habit of wearing simple military uniform was remarked upon favourably by contemporaries, especially those hostile to the ancien regime and their highly ceremonial traditions. Monarchs after Napoleon that wore military uniform (though usually a more "parade-grade" quality, as you mentioned), even though they did not command their armies, did so not necessarily to imitate him directly on a personal level but did so for some of the same reasons he did; to appear more 'down-to-earth' and less formal and high above everyone else. Especially in the early-to-mid 20th century when European monarchies were being abolished left and right because they were seen as being outdated and a relic of the past, monarchs were eager to appeal to the lower-classes and present themselves as not being quite as distant and 'elevated' as they formerly were. By wearing military uniforms instead of regal gowns, they were both a) creating a sense of solidarity with the army (which consisted primarily of lower-class demographics) and also distancing themselves from the somewhat unpopular image of high-and-mighty, haughty monarchs of a bygone age.