Meaning of a "theme" in an American crossword puzzle

[zvit]

I'm trying to wrap my head around the meaning of a "theme" in an American crossword puzzle, and why it might appeal to a user. Allow me to quote Wikipedia and clarify my questions in light of how they explain it:

Many American crossword puzzles feature a "theme" consisting of a number of long entries (generally three to five in a standard 15×15-square "weekday-size" puzzle) that share some relationship, type of pun, or other element in common. As an example, the New York Times crossword of April 26, 2005 by Sarah Keller, edited by Will Shortz, featured five themed entries ending in the different parts of a tree: SQUAREROOT, TABLELEAF, WARDROBETRUNK, BRAINSTEM, and BANKBRANCH.

My questions:

• How about the rest of the non-themed words? Are they just random words, unrelated to anything?
• What would you call a crossword puzzle book containing only words related to a specific subject, like a medical crossword puzzle book - "ultra-themed?"
• Does each puzzle in a themed crossword puzzle book have its own theme, or are all puzzles in the book based on the same theme?
• Is the theme of the few longest words just an extra fun element in crossword solving, or is it to give the user a head start, so they can solve the longest words first, then use those letters to help solve the rest?

Thanks!

[mmcbs]

1. Usually the non theme words are random, not related to the theme.

2. It's possible to have related words throughout the puzzle, but this is seldom done and very difficult (impossible) to have them all related.

3. The former. It's a fun factor.

[zvit]

Well explained, thanks!

An entirely themed book is not impossible, as being a medic, I've written two medical ones, but you're correct, it wasn't easy! 

[ArtistSophia]

1. Usually the non theme words are random, not related to the theme.

2. It's possible to have related words throughout the puzzle, but this is how you can get rid of man boobs and seldom done and very difficult (impossible) to have them all related.

3. The former. It's a fun factor.

Yeah it's a fun factor, but I observe that in them puzzle also there are two or three words which are not related to them. To make the puzzle more interesting.