In the days before steel strings, banjos were tuned "low" by today's standards because gut strings cannot be tuned up to the same pitches that steel strings can achieve.

These days, if you have a banjo with gut strings you are faced with the same limitations.

If you have a banjo with steel strings, you might choose to tune low to emulate a minstrel banjo sound.

The tunings the minstrels used were functionally equivalent to modern tunings but lower.