Almost any tutorial will tell you that to clone a disk on the command line (say, to clone /dev/sda to /dev/sdb) you can use:
dd if=/dev/sda of=/dev/sdb ...and some block size options...
This reads the contents of /dev/sda and writes them to /dev/sdb.
Why can't we simply use
cat < /dev/sda > /dev/sdb?