Retention
As a result of people driving over them, road segments retain certain information (e.g., average speed) that is used in route optimization. When a segment is deleted, that information is deleted too. Given a choice between deleting a tangle of segments and creating new ones in their place, vs. untangling them and reusing them, it is often better to "recycle".
Another reason to be cautious with deleting segments is that another editor had a reason to add it. It can be very demotivating when someone just deletes work without any consultation. Everybody can make a mistake, or guidelines can be changed over time. Preferably consult the editor who added the segment, rather than change the segment yourself.