Simply put, it is to slim down the documents. Or avoid conflicts between foreign layers and block definitions.
Example of practical application: copy and paste a block from A to B, and delete it if it is inappropriate. After modifying the block in A, copy it to Figure B. At this time, the block shown in Figure B is still the style pasted for the first time. If you use the pu command to clean up the drawing after deleting the first block, the second pasted block can be displayed normally.