The most common way to use a Proxy card is for playtesting
While exiling your own creature is generally frowned upon, youll be gaining yourself 10,000 life, which is a huge amount in any game
Slime Against Humanity creates a 0/0 Ooze creature token with trample when you cast it, putting 2+X +1/+1 counters on it, with X being the number of cards in your graveyard or exile with the Ooze creature type or with the same name as Slime Against Humanity
It takes very little to turn two counters and two spells cast for into a significant advantage
Real ones tend to show a clearer showing of where the LED light is behind the card in a clearer tone
Ideally, if you spend $35 for a Commander deck, you don't just get the deck but also good cards to grow your collection and lots of opportunities to build different decks with the same cards