Does it add content or does it also transform content?

It does both.

In Rimworld, your content from beginning to end is the story you're telling about your colony. Ideology, and to a somewhat lesser extent Royalty, both significantly change what can happen in that story.

It's hard for me to comment on specifics since I've long forgotten what's vanilla and what's added by my unnecessarily bloated mod list, but I do know that both DLCs changed up the theme and playstyles of the game a lot. Psycasting was probably the biggest thing with Royalty, but it also adds a new way to get off the rim as well. Ideology adds the most significant playthrough changes with the precepts and memes of the Ideoligions, and that can really change how that particular story is told.

I personally think they're worth it. If you're unsure, it doesn't hurt to wait until you've played a bit of the base game.

To be clear, you can enjoy rimworld base game with no DLC with no downsides whatsoever. I would personally recommend trying out the base game first just to make sure it's something you want to invest another 40 dollars in on DLC.