The queue is a running order for what plays next. You can line up several cassettes back to back, so when one tape ends the next one starts without you touching the phone.
Reorder and shuffle
Tracks in the queue can be reordered, and you can shuffle the lot if you'd rather be surprised. That's useful for a long cleaning session or a dinner where nobody wants to choose.
A simple evening
Say you have three albums for a Sunday afternoon. Put them in the queue in the order you want, hit play, and leave the phone alone. If the second album turns out to be the wrong mood, move it down the list.
Queue versus mixtape
The queue is temporary and a mixtape is permanent. Use whichever matches how long you want the result to live.
To find things to queue quickly, the Search tab helps a lot.