First of all these files are not useless. They tell you about the health of your system. System maintenance in short is System Administration, this is what I do everyday. It is the definition of a UNIX system admin.
I am not sure why you would want to compress an entire filesystem, that would be rather dangerous if it is a system directory necessary for the operation of the box. Very few if any filesystems are so full that you would need to compress everything in it.
You can do a "man compressdir" or "man compress". The compressdir command will recursively descend through subdirectories and compress whatever it can.
BUT this could be very detrimental to your system depending on what filesystem it is.
System maintenance is many things.
Performance montoring of CPUs, memory, paging.
Hardware maintenance, user adds and deletes, watching logfiles for errors and warnings, cleaning up core files and monitoring users' use of space, preventing filesystems from reaching 100% especially /var and /root and /tmp, filesytem maintenance, creating new filesystem space, adding new disks for growth of existing filesystems.
Archival of old data, maintaining a backup schedule, offsite storage of backup tapes, disaster recovery, security for boxes, building security, firewall security, etc...
These are just what I can think of right now.
One very powerful command to help you with logfiles and core files is the "find" command. You can "man find" and read about it. It is a very versatile command with many options.
Hope this helps. UNIX is very broad. It may seem very confining at first, but I know of Admins who, after 15 years are still learing something new about UNIX. I have been doing this for almost 5 years and I learn new things almost daily!
The more you learn, the more you realize how much more you don't know.