Over time while building craft you can end up with quite a large collection of vehicles in the VAB and SPH. Minor modifications to one design get saved as a new version, and before you know it you have, for example, Miner V1s - V10s saved and it clutters up the load panel.
V10 of your miner might be the best one you made, but you don't want to delete the previous verisons. Maybe earlier versions have a nostalgic feel or you messed up somewhere in the later designs and you need to recall earlier designs. What if you had the option to make a design obsolete, which would hide it from the default load menu, but still be available if you click a box marked "show obsolete designs." Currently the only options are either delete the craft file, or manually move/delete craft.
I believe that an option like this would benefit players by helping to keep their load craft options more manageable, whilst still letting them be able to reatain earlier designs.