Although the SD card is certainly the most common boot device for an RPi, it is not the only option. For example, it is possible (on some models) to boot from a USB-based storage device, such as a flash drive, hard disk, or SSD. The initial paragraphs in the "SD cards" section do not take this into account and thus might mislead the unwary reader.
Because I'm not sure how this issue should be handled, I didn't edit the page. Meanwhile, I've made a start on a page about USB-based booting: RPi USB booting.