I agree about just holding a spare set of keys and no payment etc. that is between them and the owner until soemthing goes wrong of course
If you are handing over keys, cleaning, replacing linen etc. the poc in an emergency you are effectively the owners agent a bit diffenrnet from a permanent resident paying soemone to come and clean for a few hours week (in theory this should stil be decalred as income but we all know......)
If you are cleaning after the lets for example then you would need liability insurance becasue if you caused damage or had an accident then you are liable as you are working - same as any other person doing work on a property (electricain, plumber, builder etc).
Most house insurances would not cover you and if the house is not registered as touristca aslo it may not cover the paying guests for anything in teh event of an accident, burglary etc. or the owner for any damage caused by the renters.
Of course all holiday lets have the approriate holiday let insurances don't they and all owners inform their insurcne companies who is holding the key in spain
In LM it is recommended that key holders (paid or not) are registered with the town hall and local police in the case of an empty house having a problem which has happened (flood and fire)