For me, it adds a good source of tension (barbarians) and competition (knights, defending or not), without being direct PvP combat. It also adds a variety of different special actions through progress cards (which replace development cards). Perhaps it adds a bit more randomness but it's fun to be able to drop a "power move" with a well-timed play of cards.

As someone who is a huge fan of the Cities and Knights expansion: The expansion is a huge expansion to where the entire game changes.

What I like about it the most is that instead of cities getting a second of a resource, they get a new type of resource that is used to "level up" three categories. These categories have three purposes: an opportunity for a couple victory points for each category, increases the chance of getting the new type of development card, and the most interesting purpose is that it gives you a superpower at level 3. One superpower allows you to power up knights and I can't remember another, but the last allows you to get a resource of your choice whenever you get unlucky on the die roll and don't get a resource. Easily the least fun part of Catan is when the die rolls make it so that you don't get anything, so I love this power that offsets it.

It also adds the concept of knights and barbarians. Knights are a new thing to build, and they need a grain to activate. (There's also a wall that increases the number of cards that can be in your hand before the robber affects it so the expansion adds three new things to use resources for). Barbarians attack every now and then, and the number of active knights will determine whether you win or lose. A loss means that the people with the lowest number of active knights get a city destroyed, while a win means the people with the highest number of active knights either gets a victory point (if one person) or a card (if more than one person tied). Active knights can also be used to move the robber or move or even eliminate another knight in very particular situations.

To add some detail: you roll a second die at the beginning of your turn. There's a half chance the barbarians move closer to attack, and the other three possibilities correspond to the three categories of the new type of resource and link each one with a development card. There are now three types of development cards that have different slants of capabilities. Your level in each category goes from level 1-5. Level 1 grants a development card if the first die is a 1 or 2 and the second die corresponds to the category, level 2 grants a card if the first die is a 1, 2, or 3, and so on.

The last detail is that the first person to level 4 in a category gains a couple victory points and is granted immunity from pirates on a city, but the first person to level 5 in that category can steal it for the rest of the game.

That's pretty much it, as you can see it really is a completely different game from the original.