like when does an api get stateful and what does that even mean

hey folks, i've been thinking a lot about API design lately, especially when prepping for interviews. we often hear stateless RESTful APIs are the way to go. but honestly, when does an API actually start carrying state? is it when we add user sessions, or maybe when we have to track how a multi-step thing is going? what are the trade-offs and challenges when an API gets more stateful? really curious to hear what you all think and what you've seen.