Waster hit the nail on the head but I will explain it a little more for your understanding.
1. People like to collect everything in the game, the more cards you have the more respected you will become. You get titles such as Novice, Senior, Hero, Titan, Legend etc. The higher your cards the better your title.
2. They are useful for missions because they are cards that are over powered. Cards like Gil
will help a lot in Pill Removal missions. Cards like GraksmxxT
will help in Damage Removal missions etc.
3. In Survivor
Type 2, Deathmatch and Fight Club these cards are allowed with no penalties, they are considered the same as any other card and with them being over powered they have a high advantage in these formats. They can also be used in DT to an extent. I wouldn't use any of the - 5 or - 10 cards but some - 2 cards like Rowdy
can help you out and may be worth the point reduction based on preference.
A lot of people also believe they might go CR because people believe that a lot of OP cards go CR and decide it's a good idea to invest in them. Nothing is certain however and if you are considering buying any of them I would recommend for general playing over the other options.