Why specify block size when copying devices of a finite size?

In online tutorials it is often suggested to use the following command to copy a CDROM to an ISO image: $ dd if=/dev/dvd of=foobar.iso bs=2048 Why must the block size be specified? I noticed that in fact 2048 is the [standard logical block size for CDROM images](https://www.thomas-krenn.com/en/wiki/Create_an_ISO_Image_from_a_source_CD_or_DVD_under_Linux) but it seems that dd without specifying bs= or count= works as well. **Under what circumstances would it be problematic to _not_ specify bs= or count= when copying from a device of finite size?**