Unusual use of "ne" in a phrase

I recently started 15.5 and there is a phrase "Dites-lui au revoir avant qu’il ne parte." which translates to
"Tell him goodbye before he leaves."

I assume the "il parte" is subjunctive, but why is the "ne" present - is it just a common type of phrase?

OK, I've looked back at module 13 where the subjunctive is introduced and the only sentence that uses "ne" is "Il est probable que mon père ne puisse pas venir" meaining "It is probable that my dad won’t come", so this is obviously a negative.

However, I checked in the BBC "Talk French Grammar" & it says (p180)
"A few conjunctions can also take an (optional) 'ne' which doesn't have a negative meaning: avant que,  before, à moins que, unless, de peur que, de crainte que, for fear that. "
So that answers my question!

It might be worth adding a note to the grammar text.

Yes, this 'ne' does not cause a negation and is used in writing but often omitted in speech( so I'm told).

Thanks Grace, I'll keep my eyes (and ears) peeled for any more examples!