why hs-boot

[Serge D. Mechveliani]

For recurive import, GHC suggests using an  hs-boot  file to 
"get started". 
So far, I do not know, what does this mean.
But I do not understand, why at all GHC needs any help in this 
situation.
Look:
* we write the two modules importing each other, 
* reading these modules, we understand the computational meaning
  of the whole program.
Hence, it must be compiled according to this meaning.
Why does GHC need additional help?

Thank you in advance for explanation.

-----------------
Serge Mechveliani
mechvel at botik.ru