The benefit of them moving to a GLA is likely similar to that of their moves to other main airports like BCN, FCO, BRU etc - they open themselves up to pax who aren't keen on secondary airports (i.e. a lot of biz pax) and can often get higher yields too. On a lesser note, I would also perhaps suggest that some city routes with higher inbound balance are more marketable to GLA then PIK - GLA is more of a realistic substitute for EDI and also offers better access to the Highlands than PIK.
Wrt to people saying that a move to GLA would result in a "war" with EZY/LS/Charters - This rather misses the point that FR's PIK ops are obviously in competition with these carriers GLA flights now.