To get rid of a lot of the donkey work that would otherwise have been left until the final audit
The interim audit sees the auditor confirming that systems are still the same as last year or, if not, noting the revisions and then, whether they are changed or not, carrying out the detailed tests of control to establish the efficiency and effectiveness of those controls
All of this work will alleviate the time pressure that would otherwise have obtained during the final audit
You say that the interim audit is “not an audit engagement” but you can look on it as though it were a preliminary, integral part of the final audit