Using HTTPS seems to be standard and more and more sites are making the switch and redirecting all users from HTTP to HTTPS.
However I have come across some large sites that keep HTTP and HTTPS both available and indexed in Google.
The only reason I can think of keeping the HTTP pages indexed is that older devices/browsers may not support HTTPS. I have found this out from my own testing but obviously, that number of incompatible devices gets smaller every day.
The SEO reasons to move and redirect users to HTTPS are obvious. Secure, combine link juice to one page only, no duplicate pages, save on crawl budget.
So why would sites support both?