Why would one want to change the root bridge to a different switch instead of using what STP comes up with? I would figure that STP would do a good job of figuring this all out.
Yes, because I don't want to have another switch gets the root due to missfonfig or spoofing. As well as maybe it makes sense to control that manually instead of trust STP which will do a good job of course but it depends on what's your design and goals so maybe it could make sense.
Typically you want a stable, central switch to be the "center" and root of your network; rather than it be chosen automatically based on just MAC address of the switch, or some other arbitrary switch root election.
