If you're working in a service industry role, building some level of rapport and comfort helps insure repeat business, tips, better reviews, etc. Depending on the context, some flattery is an easy way to lower some barriers. So it makes sense for an escort to stroke your ego as well as your body.
An escort who can find a aspects of a client who isn't exceptionally attractive and engage with them in a way that doesn't feel like executing a contract tends to earn a better reputation. Some banter or regular conversation when not in bed would be part of that, and a compliment or two helps with that.
Is every complement you give a customer, coworker, or family member 100% sincere? Probably not, but it helps people get along better.
Of course, any good thing becomes a negative in excess. A light compliment about a coworker's outfit helps smooth a conversation, but effusive gushing about it might get you an HR complaint. Same thing in escorting, less can be more, and the good guys have the emotional intelligence to know if they're crossing a line.