When the ideal world meets the real world, the real world wins.
When central configuration was first introduced in SAP NetWeaver 7.02, I had an idealized view of how the change processor should work. Click one button and the entire landscape would be checked and updated where necessary. You would always be certain the landscape was in good order after the change processor had run. If you had a problem somewhere in the landscape, click that button and everything would be fine.
In the real world, there are often landscapes of several hundred systems (not the half dozen that I get to play in) and administrators wanted the ability to select the systems that should be involved. This could be particularly relevant in the larger landscapes– if you know that there are only changes to be made to a specific system, you can spare the central and managed systems unnecessary work. For example, you may have skipped a processing phase in one system during an earlier run of the change processer (the system may have been down or some other problem encountered) and you only need to update the configurations there.
The option to release individual systems or selected systems (see screenshot) is already in SAP NetWeaver AS ABAP 7.4. If you want similar functionality in other releases, use SAP Note 1866268 – Create and Change Information in Change Processor.