I should expand on that... not only will usually you keep your monitor longer than any other component, it's the thing that you actually look at while you're computing... it shows you what your computer is doing, and is your workspace, etc.
At the same time, your monitor really sets the requirements for your PC build. If you've got a 4k monitor, you'll need a system that's 4x as powerful as you would if you were using a 2k monitor. Same with refresh rates — if you're doing a 120+ hz gaming monitor, you'll need double the graphics card muscle to reach a framerate that takes advantage of that.
It always makes me facepalm when I see someone build a fancy $1200+ gaming PC then put a $120 23" monitor on it.