Matthew, a defined ceiling will reduce the volume of space you need to heat/cool and provides some opportunity for sound mitigation. You can certainly do an insulated envelope without a flat ceiling, but that volume of space above where you might normally put a ceiling also has to be heated/cooled so it will affect sizing of your HVAC system. One of the best things I did in my shop when I insulated was to put inexpensive acoustic ceiling tiles in...the noise reduction was "yuge".
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The most expensive tool is the one you buy "cheaply" and often...