Are gender-segregated religious practices ever necessary?

That would depend on the religion. In our way, many of our practices are separated by gender. For example, after somebody dies, and we prepare for their funeral, it is only the women who lead the "cry songs" that help the spirit of the deceased to find their way to the next world. 

It is better to do the practices before a judge, his interpretation will be clearer!

It's tradition. And much of it offers certain, practical benefits.

Do you have examples?

Not if the elders are attracted to both boys and girls.