You can check Announcing the official release of the Visual C++ Build Tools 2015 and from this blog, we can know that the Build Tools are the same C++ tools that you get with Visual Studio 2015 but they come in a scriptable standalone installer that only lays down the tools you need to build C++ projects. The Build Tools give you a way to install the tools you need on your build machines without the IDE you don’t need.
Because these components are the same as the ones installed by the Visual Studio 2015 Update 2 setup, you cannot install the Visual C++ Build Tools on a machine that already has Visual Studio 2015 installed. Therefore, it asks you to uninstall your existing VS 2015 when you tried to install the Visual C++ build tools using the standalone installer. Since you already have the VS 2015, you can go to Control Panel—Programs and Features and right click the VS 2015 item and Change-Modify, then check the option of those components that relates to the Visual C++ Build Tools, like Visual C++, Windows SDK… then install them. After the installation is successful, you can build the C++ projects.