there are a few reasons.
first are already breeding pairs for example
a male and female 3yo murray darling carpet thats already breed would be sold as a pair because there would be more intrest in a breeding pair than two individuals.
secoand are some monogomus (pair for life) animals such as stumpy/shinglebacks are sold in pair because they buyer would feel more confident in getting two animals that are already pair'd
and last reason is expenive animals such as mangrove monitors, GTP's. Womas and BHP's can be sold as a deal for the pair so you get them cheaper and the breeder sells two reptiles instead of one which is good if they have 50+ juvi snakes to get rid of.
there would be more reasons but these are the main ones i know about