>> You don't NEED them, but if you have a lot of empty channel slots you can also see what you hear
Except for the "curiousity factor," anyone listening to these links will quickly lock them out of the scan list.
I believe the links used on Hatteras Island use pilot tones when voice / audio is not being passed on the link. The link carrier is up and running 24 / 7, broadcasting a pilot tone. The tone mutes when audio is transmitted ... then goes back to broadcasting the pilot tone.
VERY BORING, and prevents the scanner from scanning.
I have logged a fair number of this type of link along the I-85 corridor between Anderson County, S.C. and Atlanta. The continuous tone makes it very easy to find.