Usually Hull & White is calibrated to co-terminal swaptions. When asking why specifically co-terminal, I get the response that it is just a choice and it depends on the use we intend to do with the calibrated Hull&White.
But as I read more materials on the subject, the co-terminals come up almost every time when calibration of the Hull White is discussed.
I am sure this is not just a coincidence. Can anyone help me understand why we pay so much attention to coterminal swaptions for calibration?
Also how does one choose to use 20Y-coterminal, 10Y-coterminal ... or any other maturity? what is the reasoning behind?