All the time: 1. While you are in the middle of writing your file the first time. 2. When your IDE/editor doesn't see all the includes/imports 3. In language

All the time: 1. While you are in the middle of writing your file the first time. 2. When your IDE/editor doesn't see all the includes/imports 3. In languages where syntactic validity depends on semantics, beyond merely known and unknown identifiers. 4. When you make a syntax errors. Which is all the time, basically, until you've gotten your program or library to compile. 5. When you choose to format just a small segment of code, not the whole file.