To my mind, ‘good to see you’ is something you say only to someone you already know. When meeting someone for the first time, ‘good to meet you’ feels more appropriate to me, perhaps because it puts more emphasis on the meeting. But increasingly people seem to be saying ‘good to see you’ even when they’ve never met you before.
It’s a similar story with ‘How are you?’ To my ear, this is only appropriate for someone you already know. If you don’t know someone, ‘How do you do?’ is better. But nowadays, people you don’t know are very likely to say ‘How are you?’ To me, this feels slightly intrusive.
‘How are you today?’ seems to be particularly common in service settings, for example in shops or restaurants, or if someone is cold-calling you. I suspect sales people invented this as a strategy for building rapport quickly by faking intimacy.
I know these are all only conventions and don’t matter much. But this dinosaur won’t be using the new ones for a few more years.
A few observations about ‘good to see you’:
Sometimes, ‘good to see you’ seems to be used for protection from a social blunder when the speaker is uncertain about having met the person being spoken to before. ‘Good to meet you’ implies the two have never met before (a faux pas if, indeed, the two have met), whereas ‘good to see you’ leaves the door open.
Also, people will say ‘good to see you’ when they are certain they have met me before but cannot recall my name. Definitely, I have done this myself.
Further, when I was working in local government, I noticed that politicians sometimes use ‘good to see you’ as a bit of a campaign tactic. By saying ‘good to see you’ when encountering a stranger seems to suggest that the wise politician is familiar with the person being spoken to even if that’s not the case.
The young are more likely to say ‘You alright?’ ?