The crontab job clears the Twonky image cache and you can comment it or completely remove it. If you change some if the images embedded in their music files, they will only show up after you manually empty the cache, or rebuild the Twonky database, so my guess is that one advantage of that crontab job is that any replaced images would show up within 24 hours. Personally, I've moved the Twonky cache, increased its size limit (both done via RPC calls) and removed the crontab job to delete the contents at 3 AM as I've a lot of 800 pixel embedded images and want the cache preserved (so I don't have to build it up every day; that would slow the responsiveness of my control point). Whilst in there, I also changed the other crontab jobs to only occur once a week, and thus far, I've not noticed any deterioration in anything (but I mostly use it as a media server, so most other services are disabled).
Below is how I now have it set, and I'd be interested to know if anyone recommends I change any of the below tasks from weekly back to daily. As I say, I've not noticed any problems, but then I mostly only use it as a media server.
# m h dom m dow cmd
1 3 * * 3 /etc/init.d/idmap.sh dump
2 3 * * 3 /sbin/hwclock -s
3 3 * * 3 /sbin/vs_refresh
5 3 * * 3 /sbin/clean_reset_pwd
4 3 * * 3 /etc/init.d/backup_conf.sh