As @otakunet21 suggested, imagine if we just switched Lite out for the main website - it wouldn’t go down too well. Instead we can experiment with things and try them out first including getting feedback from new and old users alike without the complexity of the main website.
Anything that is a highly used feature on the main website should eventually make it’s way into Lite, but instead of just recreating the same thing, which would be pointless, we’re able to take some time to try produce a better experience first.
Something like the Calendar is a good example - it’s definitely in the works. And you could argue that it should already be there, but at the same time, not everyone’s usage is the same - personally I don’t use the calendar much at all (just a coincidence of course) other than viewing my upcoming schedule so trying out Lite doesn’t lose me that many features.