the musicians were to play jokes on each other 这句中为什么要加介词to?

the musicians were to play jokes on each other 这句中为什么要加介词to?

was/were + to 过去将来时
am/is /are + to do sth一般将来时,表示按计划,约定将要发生的事情
eg:we are to meet at the school gate tomorrow morning.
was/were +to do sth 过去将来时,表示在过去的时间来看,事情将会发生.使用这种时态时,要注意时间的参照点.在间接引语中用的比较多.
eg:He said he was to leave for Beijing next week.

这个to个人理解是动词不定式做表语，表示目的，表示“是为了互相开玩笑”。
希望看到上下文，这样可以比较容易做出更准确的判断。