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.
I agree here with Frowns.
Why would I put a Gil on my Junkz survivor extended deck?
And in survivor elo I can't have it on my deck, it's elo banned.
So, besides Fight Club, where you gain... nothing, no extra clintz or points, I can only use on survivor extended, where he's nothing along side with the big cards like Haze, Neil (talking about pillz remotion), Rowdy... you get it, right?
So... yeah, what do cards like Arno, Gil, Arkn, serve for?
(And guys, sorry for my baaad english, I'm portuguese )
Well, as of right now in official modes, just collecting and trading I guess, but it's not unheard of for the staff to "unpermban" a card, if they feel the meta has changed enough.
There are player-created events that those cards could still be used in, though. Heck, you could even start one yourself.
+1 to a lot of the above regarding its ontological issue
Gil is still at the apex of what a 2* card can do. There are still plenty of game modes and events available that allows Gil to shine. The change of the T1/T2 system to standard-extended does somewhat reduce Gil's options, luckily there is still extended 25
Ougun kyo on the other hand...