There are many stations like Sion who cater to only slow local trains but have four platforms to their names like Khar, Mahim. There is no single reason why these are in existence, like during megablocks, sometimes slow trains are diverted on fast tracks and hence these dormant platforms are used then. Some of them have been made for future provisions like parel (terminus in near future).
Can't comment on why semi-fast/ fast trains don't stop at sion, although added infrastructure (extra platforms) maybe put to use for adding more locals once cst-thane 5-6th line gets into place.