Taken from the OpenMPT wiki project:

Fast Volume Slides Applicable only when loading S3M tracks, checking this box will apply volume slides on every tick (as opposed to every non-row tick), which was the default behaviour in Scream Tracker 3.0. Normally you should not need to change this setting as it will modify note slide behaviour.

So in otherwords you need it to be a .s3m

but really, unless you have a super specific goal, there arent many reasons why a .it would be better than a .s3m besides maybe familiarity with the effects