I think local knowledge is essential. That's why I declined to transcribe some FreeREG documents from areas of the country I hadn't ever even visited or knew where they were.
Within a local area you have particular names that crop up again and again that, elsewhere, are unusual and could easily be mistranscribed simply by the transcriber not knowing that this variation of a name exists.
Even more so with making sense of local placenames. Little idioms and "the way we always write it" etc.
Anybody could transcribe some sheets .... but a local will add that extra %age of accuracy from local knowledge that's essential.
I also think that, with local records, a transcriber's got more of a sense of "ownership", taking each line a little more personally as they can visualise the village/location as they're doing it. It's personalised to them. It's loved more. You go the extra mile for your own child.