A good greeting puts people at their ease and helps them to like you (this is why greetings are as they are). As you greet them with open arms you are saying ‘Look, I have no weapons’. As you look happily at them, you are saying ‘I am happy to see you, so you should be happy to see me.’ As you fuss over helping them to their seat, you are saying 'I care about you, so you should care about me
Smiling is a universal body language signal of happiness. This has do be done genuinely to be most effective, with the eyes smiling too.
A warm greeting is important as our first impressions of people are critical for setting the way we feel about them. There is also a primacy effect in the way we will later recall the initial few moments more than the subsequent events.