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#11
You have had some good explanations of why it is important to shutdown gracefully. I'd just like to comment that the heads in an HDD are not "needles" and shutting the power down quickly does not cause physical damage of the HDD or SSD in any way. The HDDs actually use the inertia of the disks spinning down to generate power to retract the heads so no physical damage occurs. As has already been said, a sudden interruption of a write to the disk could lead to corruption of data.
If it is necessary to power the machine off abruptly, if possible, try to do it only when there is no disk activity since disk activity may mean that data is in the process of being written to the disk. Interrupted writes lead to corrupted data which is true for both HDDs and SSDs.