Perhaps there is a sense that carrying coffee or having it nearby confers the idea of productivity also. So not only are we drinking it to get us through the day's activities, but we have it with us to seem like we're busy and productive during times when we're not actually working—it could almost be classified as a status symbol.
I definitely can concur with this statement, as I, too, fall trap to this illusion of productivity when I have my coffee with me. It makes me feel like I'm doing more than I am. Similarly, when I see others' working at a coffee shop or doing their homework along side a coffee cup, I assume that they are being extremely productive and hard working.