Pentax K-1 vs Nikon D850

K1 has an alternate shooting mode that can significantly reduce noise and increase sharpness. It's called Pixel Shift Resolution. Once enabled it will take 4 shots moving the sensor by 1 pixel each time in order to capture all color information for each pixel instead of distributing it over 4. A visualization can be found here.

Pixel Shift Resolution shooting mode takes longer as it's taking 4 shots instead of one and requires a couple of seconds after the shots have been taken to process the images into one so it won't work for sports or action, but it's great for landscape photography.

You can potentially do night photography without a tripod by using a fast prime lens wide open with in-body image stabilization.

K-1 has a cross-tilt LCD.

It's not the camera alone; it's the camera and the lens together. Pentax's best lenses are the FA31/FA43/FA77 Limited series. There is no other camera + lens combination that can compete with the K-1 + one of the FA Limited lenses on a price/performance basis. It is not even a close competition.

Great searching through picture libraries by location.

There's an LED that shines light on rear controls, there's another one that illuminates the lens mount, and an LED that highlights the card bay and remote release port. These can be really helpful in the dark.

When using manual focus lens, you can turn on focus peaking to see which parts of the picture in in focus, as there will be a colour (by the default settings red) "painted" to tell you that that part of the picture is in focus

Also know as focus stacking, this camera is able to do focus shift photography on its own after the set up, thus it will not be so tedious in terms of capturing the photo.