
I don't like that though,your pootling along at a fair distance behind a car and they decide to drive near the curb to let you pass,even waving you by sometimes and I don't feel like overtaking at all just happy to pootle
I think they panic if they see a bike in their mirrors
I do sometimes do it out of boredom, or loosening up after riding in a straight line for a while, or if I've been through some water/mud and want to make sure the tyres are clean. 
in reality any friction is going to cause heat so it cant hurt, but I am sure I read a comparison years ago that building up your speed and braking on back roads before going all out warms your tyres better than a blast up the motorway, which is what plenty of people think warms your tyre up but in reality the sides are fairly cold in comparison.