Man, read carefully and understand what you are doing, to avoid mess.
catpach is necessary to apply the changes to the data dictionary, necessary for some fixes in the patchset
To be more detailed, when you apply a patchset on the database, the installer replaces some executables with new ones, where bugs are fixed. Also replaces scripts, whatever it needs.
Some of the fixes however, requires changes in the data dictionary as well ( for example there is a field in the DD, which is varchar2(30) but ti fix some issue, the field is changed to varchar2(45) or additional field is added, or additional tables or a view definition is changed... whatever)
So the newly installed binaries will not work with the old DD, and here catpatch comes.
Now, on the client, you do not have a database, just a binaries to be replaced, so generally you run the installer, it recognizes that you have only client installed and replaces what it needs. Catpatch has nothing to deal wit that
BE CAREFUL!!!
if the server machine and the clients are different platforms ( say unix/linux server and Windows clients) you cannot apply the server patchset to the clients. You will need to download a patchset for the client OS.
Also full suport to Jeny, read and follow STEP BY STEP the patch applying instruction

Regards