The more you get into landscape photography, the more you’ll find ways to perfect your images in-camera. One thing that I try to get right every time while out on location is a perfectly straight horizon, as it’s one of the first things people pick up on, and correcting it in post necessitates cropping your image. This is not ideal, especially when the problem can be easily fixed.
Some cameras now have a built-in spirit level which can be accessed via the viewfinder or live-view screen. These are great but I still find having a spirit level attached to the top of my camera to be most reliable.