Why do people say 慢点儿阿("man dianer a") when you leave their shop?
慢走 is a polite thing to say usually used for someone who is leaving, has two implicit meaning:


I don't want you to leave, so please leave slowly, so I can stay a little longer with you.
Don't hurry, take care.


慢点儿 has the same meaning, but not as formal as 慢走, and sounds more affable.