There's a street I commute on where the local authorities decided to paint bicycle stencils every so often down the middle of it.
Thank goodness that they are mostly faded away by now, because nobody knew what they were for.
For reference, it is 25mph, one lane each direction, with a double yellow line and traffic lights. There are businesses, parallel parked cars, and sidewalks on both sides. Other streets in town do not have the markings.
Wonders:
Were the local authorities simply on drugs and trying to "do something"?
Could be, I just don't know. 
Are the markings simply a friendly reminder to drivers to look out for bikes?
Yes
Do the markings have any legal meaning (for cars or bikes)?
Perhaps. See your city's traffic code.
Are the markings meant to encourage bikers to take that particular street?
Yes
Do the markings mean bikes should take the center of the lane and not let cars pass?
They alert you that bikes may need to do this.
Personally, I ignore the markings, as does everyone else.