I assume they allow Duos into solo/duo queue purely because of the bot lane dynamic of having 2 players there, and since supports are usually not a primary carry role, it allows supports to have a primary carry role as their partner along with the person they'll be interacting with the most during the laning phase. Without duo queue all supports would then be stuck with a random ADC which can be frustrating at times... Obviously that's just my theory on it, and obviously it gets used in other ways since it isn't a requirement that you have to queue up for ADC and support, but you know... has the possibility of making a support's life easier.