Lately I started patching a client’s database (22.214.171.124) to the latest PSU (180417). This is a RAC environment with streams and all kind of other features, so over the time we hit quite a lot of different bugs. When we planned this PSU (we installed the bundle patch version), we added about 7 one-off patches (some are recommended by Oracle and some we had to add because the bugs affected us quite badly).
Oracle database is a very complex software, and as we all know, has bugs. Oracle tries to address these bugs, and fixes a lot of them all the time. Except specific fixes, they publish patchsets (which might introduce more than only bug fixes) and PSUs. If you are not aware of these patch types and how it works, you are welcome to read my post about it here.