Question

为什么要用when are we going不用what time are we going？

Answer 1

两者有一点区别，when 是泛指什么时候，what time 指具体的几点钟，所以When are we going?意为:我们什么时候去? What time are we going ?意为:我们几点钟去？

Answer 2

when 和what time，虽都提问时间，
但 what time表的时间更精确！
When did he come back？他什么时候回来的？
What time did he come back?他几点回答的？