I always clean before a holiday, but it's nothing I wouldn't be doing anyway because of a) the regular cleaning schedule b) practicality or c) common sense.
I also have cats so I either have a pet sitter or family member coming in daily to feed the animals, water the pot plants, pick up the mail, put the bins out/in on bin day and I don't want them coming into a bomb site.
Washing: I'll be bringing two weeks worth of washing home with me, so why would I want to give myself a three week backlog of washing to catch up on, along with the new washing created once I'm back?
Rubbish: even if it doesn't smell, who wants rubbish sitting in their house for two weeks while they're away when you can just make sure to put the bins out last thing before you leave?
Beds: I love coming home to fresh bedding - there's nothing better after feeling a bit grotty from all the travelling, than having a long hot shower, putting on clean PJs and getting into fresh sheets. Also, unless I changed the sheets the day before going away, those sheets will have been on the bed for a week or two already ... why would you want to leave two week old sheets on a bed for another two weeks and then come back and either sleep in them again or have to start faffing and changing sheets?
Cleaning: why not deep clean before you go and come back needing to do a "maintenance" clean before going back to the normal cleaning routine, rather than a normal once over before you go and then needing to do a deep clean when you get back amongst all the other post-holiday chaos?
General tidying: Unpacking creates a load of mess and clutter - sorting out washing, random holiday stuff, souveniers - not to mention having to sort out other stuff like the post build up, getting in a quick emergency shop from the co-op until I can do a big shop, etc so why not leave the place tidy and clutter free before you go?