Why use apt-get upgrade instead of apt-get dist-upgrade?

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http://askubuntu.com – I usually use apt-get update && apt-get upgrade to run my updates and upgrades instead of the GUI because it seems to run more quickly. However, I've noticed lately that I often get a message that one of my upgrades was held back. I then usually run dist-upgrade to run it through and it works fine. As far as I can tell after reading this question and its answers, dist-upgrade does all the same things and then some. So, my question is: Why use apt-get upgrade at all? Why not use apt-get dist-upgrade all the time? Why does apt-get upgrade even exist? (HowTos)