Because ideas always seem to strike when you're least expecting them.
I'm prone to daydreaming while doing repetitive daily tasks, so most of my ideas seem to strike when I'm doing housework (which makes a really convenient excuse to stop doing house work), while driving (don't worry, I don't write and drive), and in the shower (I swear, I'm going to hang a small dry-erase board in there).
I've gotten to where I have multiple writing surfaces scattered around the house. Dry erase posters and a magnetic white board (filled with magnetic poetry) scattered along my office walls. A dry erase poster on the front door, a note pad next to it, and various new or half filled notebooks in the living room, dining room, and bedroom. (If I ever own a house, I may invest in that dry-erase paint. For my office walls, at the very least.)
Where do you get most of your ideas? How do you deal with the random Muse Strikes? (Notebooks, iPads, going retro with voice records?)