Music, lights, TV, even the christmas lights.... I got no concrete answer, but I have a hyopothesis: They do it to attract attention, it does not look empty, so viewers might linger there expecting someone to be home. It is alittle hard to imagine that they all have though that through, but they likely do it unconsequently, a habbit that sticks, the newer tenants just do after the older tenants.
A proof that it is actually working, is that an empty B1 living room is very often high up on the thumbs........... (compared to an empty B2 for example, wich seldom attract much attention. I've seen it over time, so it is indepent from particular tenants being more "popular")