Bore spacers are used in Directional Drilling or Horizontal Directional Drilling (HDD). Directional drilling is often used to run utilities (power, telecom, fiber optics etc) under obstacles like roads, lakes, and large structures. This method is trenchless and allows the contactor to have minimal impact to soil.
Think contractors installing utilizes under major highways. The last thing they want to do is to stop traffic, rip up the road, install the utilities, replace the road etc. With structures like lakes, it allows them to install utilities under them vs thru them, above them or around them.