In their early appearances, they were simply in their street clothes. This was fine for the initial "monster of the week" stories, but it quickly became impractical.
Here's why they needed outfits:
* Visibility: The Fantastic Four often fight in public, and their powers are visually striking. They needed something that would help them stand out and be recognized, even during a fight.
* Protection: Their powers, while amazing, also made them vulnerable. The Thing's rocky skin was tough, but not impervious. The Human Torch's flames could burn him if he wasn't careful. Their outfits provided some basic protection against the elements and minor injuries.
* Branding: As the Fantastic Four became more popular, their outfits served as a symbol of their team and their heroism. They were a powerful visual representation of the group's identity.
In essence, the outfits were a natural progression for the team, moving from a group of scientists who gained superpowers to a recognizable superhero team. It's also worth noting that their outfits, particularly the Thing's, have gone through many changes and iterations over the years, reflecting the evolution of the characters and the artistic style of the comics.