One of the biggest hurdles Linux faces, with regards to the average user, is simple — people either don’t know about it or they don’t have the skills to load it on their computer (although, considering how easy it is to install Linux, I always question statements like that). Beyond that, the average computer doesn’t always have supported hardware for Linux (but that also is becoming less and less the norm).
What a pre-installed Linux machine (one from, say, System76 or ZaReason) eliminates is all the guesswork you must do to know if a system will work with Linux. If you buy a machine from System76, you know that when you unbox it, that system will work seamlessly with Linux. You also know that, should you have a problem, you can place a phone call and speak to an actual person who not only gets Linux but can also help you resolve your issue.