the request URL and body can be parameterizied only if using callback?

According to 'https://community.hpe.com/t5/LoadRunner-and-Performance/Testing-asynchronous-business-processes-with-LoadRunner/ba-p/6029011#.WD6bl9J96mw',

The request callback is called before each request is sent. This allows you to modify the sent request (for example, it allows parameterizing the request URL and body).

Q: That mean the request URL and body can be parameterizied only if using callback?

For example, 

web.customRequest(

{...........

body:'ViewState=xx123yyy'}

Q: Can I just use parameter to replace '123' in body instead of using the concept of Callbacks?

Thanks a lot for the help. Hope that I have explained the question clearly.