When people talk about Nexus devices, one of the main things they cite as an advantage is that they run “vanilla” or plain android. What makes it more desirable than, say, Samsung or LG’s version of Android?
Since Android OS is open to customizations, the tweaking done by manufactures and providers is extensive. The difference with Nexus is its vanilla, meaning their no tweaks that are done to the OS. You get your newest feature set and security updates straight from Google as soon as they are available. They are well tested and insures that your device is protected and up to date on its feature.