They are probably teaching it because the AIM recommends it, see Table 4-1-1.
See this link: http://blog.aopa.org/asfblog/
I'm not the only one that thinks taxi announcements in day / VFR is excessive. Notwithstanding AIM recomendations. Apparently this started as a "fix" for runway incursions several years ago. I didn't hink that this was in the AIM when I was learning to fly.