They are recommended because they know and understand Gucci (unlike many other services). We are asked for recommendations because Ebay and other online sites need written confirmation. I have never needed to use their services and have said so.
Some Gucci items take a long time to research, Gucci has one of the most complicated histories ever and has had idiosyncratic methods of doing things. I have no idea of the workload or MO of Castira but I know that sometimes I need a week or more to check on certain vintage logo patterns or a zip only used by one factory for a month in 1985 on a single model.