1. Code where the granularity of state change is smaller than a function call. Sometimes you actually have to step through things one instruction at a time, and I'm lucky enough to have such problems to solve. You can't debug your assembly with printf(), basically[1a].
2. State changes that can't be easily isolated. Sometimes you want to log when something change but can't for the life of you figure out when it's changing. Debuggers have watchpoints.
But... that's really it. If I'm not hitting one of those I'm not reaching for the debugger. Logging is just faster, because you type it in right at the code you're already reading.
[1a] Though there's a caveat: sometimes you need to write assembly and don't even have anything like a printk. Bootstrap code for a new device is a blast. You just try stuff like writing one byte to a UART address or setting one GPIO pin as the first instructions and hope it works, then use that one bit of output to pull the rest up.