Describing someone as 'hot' or generally commenting on their appearance when it isn't relevant is a form of objectification. A woman (or man) is at an event to discuss ideas or their company and the first thing someone says to them is about their physical appearance. This is very discouraging to people who have it happen to them very frequently.

Assuming you are a man, imagine being at an event and someone coming up and saying something like, "Woah get a load of Fabio over here". Being compared to Fabio might be a compliment in an objective physical sense, but I'm sure that isn't what you want to talk about when you are trying to get funding or make connections.