This is a very interesting redstone utility block. You can configure it to look for specific types of entities and emit a redstone signal when one of those selected entities enters it's scanning range.

This allows you to set up sophisticated traps or other triggers in the world. On a mundane level you can use this to open a door to your base as you approach and not let it open when monsters approach. So it can be safer than a pressure plate.

When you break the block the configuration is lost and must be re-configured when placed again.