You can basically find the information here, there is also a fair amount of other relevant information on the advantages
ASP.NET Web Site Project Precompilation Overview
Precompiling an ASP.NET Web site project provides the following
advantages:
> Faster initial response time for users, because pages and code files
do not have to be compiled the first time that they are requested.
This is especially useful on large sites that are frequently updated.
A means to identify compile-time bugs before users see a site. (You
can also accomplish this when you compile a project in Visual Studio.)
The ability to create a compiled version of the site that can be
deployed to a production server without source code. This makes sure
that that people who have access to the production server will not be
able to view the source code.
You can also find similar questions here
What is the advantage of the ASP.NET precompilation?
What effect does the new precompile during publishing option have on MVC4 applications?
And some related information here
Precompiling Your Website (C#)
Because the pages must be automatically compiled when they are first
visited, there can be a short but noticeable delay when an ASP.NET
page is requested for the first time after being deployed.