As for the reason for them, consider the first two stanzas of the evocative poem BOUTADE, by P.A. de Génestet (1829 - 1861)
O land van mest en mist, van vuilen, kouden regen,
Doorsijperd stukske grond, vol killen dauw en damp,
Vol vuns, onpeilbaar slijk en ondoorwaadbre wegen,
Vol jicht en parapluies, vol kiespijn en vol kramp!
O saaie brij-moeras, o erf van overschoenen,
Van kikkers, baggerlui, schoenlappers, moddergoôn,
Van eenden groot en klein, in allerlei fatsoenen,
Ontvang het najaarswee van uw verkouden zoon!
( English)
O land of manure and mist, where man is wet and shivers,
Through-soaked with humidity, with damp and chilly dew,
O land of unplumbed bogs, of roads resembling rivers,
Land of umbrellas, gout; cold, agues, toothache, flu.
O spongy porrige-swamp, O homeland of galoshes,
Of cobblers, toads and frogs, peat diggers, mildew, mold,
Of ducks and every bird that slobbers,splutters,splashes,
Hear the autumnal plaint of a poet with a cold....
Yes, well, it IS called The Low Countries for a reason. It's low, near the ocean, and flat, and so rain water doesn't run off it as well as in places like Switzerland. The result is mud. If many people are living by farming ( typical before 1800), they'll have to be working in it. Being in northern Europe, the mud will often be cold. Maybe even icy. What to do? Well, now we have rubber boots, but circa 1700 people wouldn't know about them yet. Those vulcanized rubber ones wouldn't come in until after 1890. Leather is not very waterproof. It can be made less porus by greasing it well and stitching it tightly, but eventually standing in mud in leather shoes soaks them. And walking around in soaking wet leather shoes can wreck them. There's also the problem of making the shoes protect the feet from damage- again, leather can be too soft. At some point, somebody tried making sturdy mud-resistant shoes out of wood. Perhaps it is not a coincidence that the oldest surviving pair of clogs is from around 1250 AD in Rotterdam.
However, even though the Low Countries maybe had the greatest need for wooden shoes, their distribution was much wider. Klompen/ clogs/ sabots were also found in France, Germany, Belgium...really throughout most of Europe: many countries that also have muddy regions and where farmers need to protect their feet.
Along with clogs there were pattens . These were essentially wooden platforms that you could put under your regular shoes to keep them up off the mud. These are likely what Genestet calls overschoenen in his poem: though "galoshes" now means a rubber boot for pulling over regular shoes, the word once meant pattens and leather shoes with wood soles. In England wooden-soled shoes were good for hard work in the northern Lancashire mills, and eventually these became the typical English clogs, made not so much for standing in mud as tolerating the hard wear and tear of a factory job.
As the Netherlands have become less about farming and fishing, more about high-tech, the percentage of the population that wears clogs to work of course has dropped from what it once was. In 1919 over 8,000 people were employed making clogs: in 1977 that had dropped to 475. But I can't speak to the industry now, don't know how many tourists buy a pair of tiny clogs for their keychains or buy full-size clogs to hang as decorations on their walls.