I’m a prop trader (at a firm) and I wear t shirts and cargo shorts to work

Most prop trading guys never have/never will be fashionable. There's always value to going to work looking good, especially if you work in an area that's more professionally attired.

After work drinks will leave you open to meeting more people, you have the optionality of going somewhere nice without going home first and changing (though once again most prop guys own very few fashionable items), and ofc you'd also probably be seen as someone that's more presentable than others at work. Definitely more likely to help you in your general career, but ofc as prop traders you don't really think about that all that much

Yes looking like a Pokémon go hunter may be comfortable, but way more upside in fitting your wardrobe with some smart casual clothes. (No form of cargo pants fit in this category).