Couple of reasons to use this feature:
Availability has changed
There is an update to shout about (e.g. new specs, layout)
Your disposal has been on the society for a while and you want to remind agents about it.
You are able to recirculate disposals once per fortnight
To recirculate click into the Disposal > Marketing > Publish Options > Society > Prepare Email & Recirculate:
Success - you just recirculated your disposal to the society!