I think the question "Why would anyone want to do this?" was not directed at you, King, but towards the original poster who asked about using a 3" shell in a 2 3/4" chamber, btdst. You can usually determine this by looking at who the poster is replying to, inside the yellow highlighted block above the text.

This topic is not about using 3" shells in 3" 20 ga. chambers, it's about using 3" shells in 20 ga. 2 3/4" chambers.

SRH

May God bless America and those who defend her.