In large project usually there are separate teams who work on the top of the chip and on the IPs.
During the project life cycle there is a constant talk between the 2 teams. In order to give the "top" team the ability to see real timing paths that need to be fixed, the IPs team make ILM of the IPs and reflect the IPs interface to the "top" team. Now the "top" can either decide to fix problematic paths by itself or tighten the timing budget given down to the IP for those paths.