If I want to check my abc.xyzblogger.com site on Alexa, it will directly show me stats of xyzblogger.com instead of abc.xyzblogger.com
And many people say that subdomain visits are not counted on Alexa.
But I have tested this with my other blogs with its subdomains and it works.
So it might be some bug or something else but Subdomain visitors will increase your root domain rank.
So you can say subdomains increase your Alexa rank.
And of course subdomains will only increase your alexa rank when they themselves have good traffic