It's not used for a "you as well" effect, it's used more to emphasise the personal aspect of the statement, usually to enhance the amount of affection it conveys; you're letting the person know that you mean them (as a corollary, it's often said that if you use a person's name when you communicate with them, you can increase feelings of rapport, closeness, trust, etc.).
You can even go a step further in the addition of personal references:
You have yourself a nice day!
Although, the above can also be said sarcastically and somewhat aggressively, and the personal references may serve to subtly enhance the conveyance of a feeling of dislike or even a threat.
We may interpret this (via a word change and some punctuation), in a grammatically correct way, as a sort of command or order:
[You] take, for yourself, a nice day (i.e. the experiences that would lead to one enjoying the day)!