This is one piece of gear advice worth taking seriously, and it's a common first-session mistake: turning up in perfectly good running shoes.
Running shoes are built to move you forward. They have a raised, cushioned heel, a soft flexible sole, and a narrow platform, because running is a repetitive forward motion. Pickleball is not. Pickleball is short, sharp, sideways — split-step, lunge to the kitchen, push off hard in the other direction.
On a lateral push, a running shoe is working against its design: the soft, raised sole is less stable side to side, and an outsole made to grip going forward isn't built for sudden sideways stops.
Court shoes are built the opposite way: a flatter, wider platform, a stiffer sidewall that resists rolling, and a herringbone or modified-herringbone outsole that grips in every direction rather than one.
There is also a practical reason venues care: many indoor courts require non-marking soles. Check your venue's rules before you go.