This piece acts as a riser to get your terrain up to the level of the sidewalks. You can mount buildings to these blocks and add other accessories to them and move them around as needed to create new layouts every time. You could use these to create parks and green spaces in your towns or other terrain features.
This riser covers the corner of the roundabout as well as 3 other crossing blocks.
See the layout image to determine which risers you need and how many you need.
Note: Depending on how carefully and accurately you applied your sidewalks, some of these risers may require a bit of sanding of an edge or two to make them fit properly.