If you have existing .jar files from a Java application, and you need to use
that existing business logic or rewrite it, wouldn’t it be nice if you could
just leverage the existing package? 

You can with the JBImp utility. 
I recently had to do this, and it was a breeze.  I ran the utility on my .jar,
and out came a managed .dll that I subsequently referenced and used in my .Net
app.

According to the documentation,
the utility actually converts the Java IL to MSIL at the bytecode level.  It
sure did make the reuse picture a lot prettier.