It is indeed a small package. It only contains TODO, README and postinst files. It triggered the lintian empty binary package warning until I put 'empty package' in the long description. I guess it'd be up to the FTP masters.
Essentially it's simply a package that links mysql_tzinfo_to_sql and tzdata together using a trigger.
When mysql-tzdata is first installed and then when tzdata is updated the postinst script is run, which loads the mysql_tzinfo_to_sql output into the mysql.timezone_* tables. That's done either as debian-sys-maint or by running a bootstrap copy of mysqld (depending on whether mysqld is already running).
This ensures that when timezone information gets updated the update of tzdata automatically updates MySQL's copy too, which should keep them in sync. That makes it simpler to update timezones, and makes it less likely that MySQL's copies are out of date. Being an optional package, people will only get this behaviour if they choose it so they could still manage those tables manually if they wished.
Since the only way I know of to do such a trigger is to use a package, it seems that although it's a small package it's also necessary to do such a task.