I am new to Audacity and I would like someone to clarify the potential damage the following (from the manual) refers to:
“If playthrough is on, the audio interface will try to play what it is recording then re-record it, creating an increasingly loud series of echoes that may damage your equipment”
Thanks!
Personally I think that actual damage to equipment is unlikely (but not impossible *1), though if the volume is turned up high your ears could be ringing for a while.
I think the point the author was trying to make is that “acoustic feedback” from the speakers to the mic is uncontrollable and can be extremely loud, so should be avoided.
*1 Speakers and amplifiers can be damaged (burnt out) by playing sound at excessively high levels, so don’t do it. 
Thank you for your opinion!