Interim plans are useful because they can demonstrate, very quickly, how far off course your project might be. This allows you to intervene and keep the project moving forward. Documenting an interim plan is also useful because they provide an updated, at a glance look at when the start and finish dates are within a project. These dates are un-muddled by other information baseline plans contain such as durations or cost estimates.
Interim plans can also be useful for documenting future projects. If you can see how long the actual project took in the past, then you can project how long it is likely to take a similar project in the future.