I am pretty new to msbuild and aspnet_compiler.
I am using aspnet_compiler to compile web application project. Now I just saw the MSBuild.exe and noticed that its builds my website into the /mywebsite/precompiledWeb folder. Now why do I need to use MSBuild.exe? Can't I directly use the aspnet_compiler to see if my website can be built properly?
(Not sure if I explained it very well).
You are correct you can manually invoke the aspnet_compiler.exe tool. But better would be to use Web Deployment Projects to help you do this.
msbuild.exe is usually used to build projects and its dependencies. When you have a web application project with a project file, vs can use this to build not only your web project but all the dependencies involved. This would only build your source code files, not the markup files (.aspx,.ascx,.etc).
aspnet_compiler is meant to build a web site project that doesn't have a project file. This said, you can also use it to make sure your markup files are also built for your web application project.