Residency card
From the EU you don't need one, but it can be handy - our only use of a Gestor so far though.
The residency card was a document that was optional for those from the EU as it counted as an ID card. Technically you didn't need it and this is the one case where we had a gestor involved since my wife's work arranged her residency card (which seemed to be a longer process which involves queuing).
The residency card was switched to a 'resident's certificate' which you technically do now need even as an EU citizen living in Spain. This is obtained from a dedicated office of the Guardia Civil (the Spanish Police, as opposed to the Mossos d'Esquadra - the Catalan specific police, or the local municipal police) and brings together your NIE and address into a small green credit card-sized document.