The games included with the PlayStation Plus Extra subscription are not permanently yours to own. You can continue to play them as long as you are still subscribed to PS Plus Extra (or Premium), and the games are still included in the collection, but if either of those things changes you will no longer be able to play them without buying a copy. Think of this as the difference between watching a show that is included with your Netflix subscription and buying a DVD so you can watch it whenever you want.
If you intend to keep hold of your PS Plus Extra subscription then there is probably no point buying the games now, but the option is there for you if for example you decide not to renew your subscription but want to keep playing.
A similar thing happens with the monthly games. In this case so long as you claim them while they are available you can continue playing for as long as you remain subscribed to any level of PS Plus, but you still have the option to buy permanently so you can keep the games even without a subscription.