Tapping into basement ductwork
Apologies if this is a naive question, but why is it always recommended to tap into heating/AC ductwork from the top? I want to tap into rectangular ductwork in my basement so as to get air circulating in a room I am building there. It would be so easy to cut into the side or bottom of the existing rectangular ductwork, yet all posts on the internet I have seen say cut in from the top, without explaining why. They only say it will be difficult due to the proximity of joist etc. and indeed in my case any cutting into the top of the ductwork would be extremely difficult. Is top-tapping simply so as to avoid reducing headroom or is there a crucial technical reason I cannot see why taps are always made from the top?
Thanks for any help.