Why are there two queues? "Next in Queue" and "Next Up"?

[momonumana]

I find those two queues quite confusing and unintuitive. What is the purpose of having those two? When I play an album it gets into "Next Up" if I then want to add some tracks to the queue they get into "Next in Queue". And then they are played before "Next Up". But naturally I always want things to be added at the end of the whole queue.

Please Spotify, improve the Play Queue. A single main queue where everything gets appended at the end if "Add to Queue" is chosen. And if I choose "Play" then this either replaces the queue, or better just adds it in front of the current queue. But not into an extra queue that is played after the actual queue. Very confusing. 

Great of course would be three options. Play Now, Play Next, Add to Queue. And it always adds at the top, inserts at next, or appends at the end.