The app makes it possible to check how many rides you have left, how many new rides you can store on your ACTV ticket, when you validated the ticket for your current ride, etc. just by scanning the ticket with any Android NFC-equipped smartphone.
You don't absolutely need the app if you travel mostly by water bus, because you can always check your ticket at an ACTV stop instead. Still, being able to check your ticket anywhere is convenient, the app's English-language interface is nicer than the little readout on the ACTV's ticket scanners, and the app is free.
For land buses, the app is incredibly convenient, because you can't check your ticket without boarding a bus. Having Imobber on your smartphone will save you from having to remember what kind of fare is on each ticket and how many rides you have left. This is something to keep in mind if you'll be taking land buses from Mestre to Venice's Piazzale Roma or on the Lido di Venezia.
Screen capture: Andrea Fontana.