You didn't mention the purpose of the PC. Gonna assume you're talking a gaming build.

Like almost anything, more money invested gives you increasingly better performance at an eventually diminishing rate. Some examples: higher/more consistent framerate in demanding games, higher visual setting options, higher resolution, the option to run multiple monitors. More money would give you a better opportunity to keep the rig relevant or "future proof" for longer by purchasing newer and/or higher quality tech. You'd open the door for better quality/larger volume storage drives (ie: 1TB SSDs). You could also choose to spend on higher quality peripherals like your keyboard/mouse/headphones/speakers etc. Virtual Reality seems like the next big thing, and Oculus has recently shown it's not exactly going to be inexpensive to enter that tech world, so there would be another cool, new future tech you would have access to with more disposable money.

If you're going to invest more in the rig hardware, say you don't want JUST 1080p resolution games at 60fps refresh with all settings maxed out, maybe you want 4k resolution and faster refresh? Now you're talking about not just a higher quality video card to display those demands, but also needing a higher quality monitor that can support those display settings, so that can be a significant part of the investment if you desire that kind of technology.

You need to consider what kind of user you would be and what kind of technology you consider essential to your experience, and you're not going to know that until you do at least some research or explain more about what exactly you are seeking. You may benefit from browsing pcpartpicker.com's "Completed Builds" section. People post the build, but often include some reasoning behind their decisions and they type of performance they were purchasing for vs what kind of performance they ended up with. May help you start to figure out what you do and do not consider essential.