There were 6 twinsets (2 FCA pairs, 1 FYA pair, 3 FIAs) on that 6V08 move with 66030 hauling. I suspect they were for temporary storage (might well return to traffic as and when required). The FCAs / FYAs were last used on either the Onllwyn - Mossend coal container workings (which no longer run) or the Hardendale - Margam containerised lime trains (which still run). The FIAs were from the set used on the short-lived Tilbury 2 - Trafford Park intermodal working which ceased running in August (that set was mainly formed of FIAs displaced from the Scunthorpe - Hayange channel tunnel steel trains).
The reason for moving them to Exeter was probably to clear siding space in the yards in South Wales. I seem to remember some HTAs being stored at Exeter, in fact I think they might still be there...