char(n) doesn't reject values that are too short, it just silently pads them with spaces. So there's no actual benefit over using text with a constraint that checks for the exact length.

So even if you're using pure ASCII, you're supposedly better off just setting the character encoding appropriately and using a text or non-limited varchar field.

That said, I thought the main benefit of a char field (and its padding) was that you could use it in select instances where is was useful to have specific length records. E.g. if all the other fields of the table are fixed length, every record can be fixed length if you use a char(n). I'm not sure if that's a benefit or not to modern databases anymore though.

Some RDBMS's don't store the string's length with each value for fixed-width character fields. So a fixed-length field may still be preferable if the space lost to padding amounts to less than the space lost to overhead.

That distinction doesn't apply to PostgreSQL, because it stores a length along with the value even for fixed-with columns.