Whiteboards are white, so your thoughts are clean.
By clean I mean away from other thoughts; distractions. Being white makes your eyes feel comfortable and your mind to be focused only on the color lines that contrast on the plain white. There is a reason chalk boards are being -slowly but surely- replaced on some schools: the dark background it annoying to your mind, which prefers some light color.
Whiteboards are big.
I know, different sizes, but in the end, they’re bigger than a paper sheet, so you can see things from a distance; something that makes easy to think “out of the box”. Sketching ideas on a paper perhaps is more comfortable in some cases, and since the pencil/pen is smaller than a marker (the lines it draws), you can get more content in a sheet of paper than in a whiteboard, but being with a collapsed now-gray sheet of white paper is not good.
You can easily erase them.
With a whiteboard eraser, with your hand, with toilet paper, with your sleeves, etc. you can make the board go white again to write some more stuff, to correct some things, to change, to think different.
You may use whiteboards for notes.
A small/medium-sized whiteboard is perfect for writing notes on, since its size lets you see the note from a distance and even lets you read it without getting close to it. Maybe this is part of the ‘size’ reason, but clearly you could use this instead of your phone notes, perhaps.
Finally, whiteboards’ markers come in various colors.
Having a white board with a black marker is great, and the contrast make you be focused, but having at least 2 other colors (3, Total) gives you different ways to organize yourself, your thoughts, notes or whatever you’re using your whiteboard for.
Whiteboards are, in all sense, the most useful tool your mind may use.