Singing in a children's choir for ~5 years when I was a kid. We were really, really good (traveled all over, sang at the Llangollen International Eisteddfod).
Overall, priceless for ear training, pitch, balance, phrasing/breathing, intonation, harmony, structure, intervals, dynamics...
Singing pieces from various cultures/nations was a huge advantage as well. Not only were the pronounciation of the languages educational, but becoming familiar with how a Chinese SATB choral work differed from a Czech SATB piece differed from an African SATB piece... which culture preferred which intervals, scales, perfect 5ths...
Was also helpful for me when I started to accompany singers as well. I understood articulation, breathing, head/chest voice, different languages, bell-canto...