They're delightfully boring, are readable at smaller sizes, have a good Unicode support, don't seem to have ligatures so i don't have to bother disabling them (personal preference) and are completely free. Plus they can be used to work with code that has large information density (think legacy Java apps) without causing strain on my eyes due to their simple look.
My own favorite programming fonts of choice at the time are either Liberation Mono (https://www.programmingfonts.org/#liberation) or Cousine (https://www.programmingfonts.org/#cousine) the latter of which builds upon the former.
They're delightfully boring, are readable at smaller sizes, have a good Unicode support, don't seem to have ligatures so i don't have to bother disabling them (personal preference) and are completely free. Plus they can be used to work with code that has large information density (think legacy Java apps) without causing strain on my eyes due to their simple look.