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This step is necessary to prevent any discharging, arcing, friction/static charges from damaging any of the extremely delicate and intricate parts of this (or most any) electrical system. In practice, you should even be wearing an anti-static band or be working on an anti-static bench even with the battery disconnected so your body doesn't create any unintentional charges.
One of the first things you should learn in electronics repair and and electrical in general is to cut the power source whenever possible before performing any work. Some systems have schedulers/triggers set that will wake the system up (even when closed) to perform updates and other maintenance tasks as to not eat up CPU and RAM during 'peak hours'. The last thing you need is to have your $2,000 laptop turn on while pulling an SSD (which, correct me if wrong, isn't a plug-and-play based device) which could do some serious corruption and/or damage; when all you had to do was peel a sticker and plug.
Hope that helps!!
~the more you know~