Traces and spans can be extended from or added to existing logging, but they aren't the same.

Logs are point in time, spans are a duration. Logs are flat, spans have a hierarchy.

It's the difference between logging a message in a function, and logging the beginning and end of a function while noting the specific instance of the fn caller.

If you have many threads or callers to the same function that difference is critical in tracing causality of failures or any other type of action of note.