I don't know how you come to that conclusion. How would the manager or anyone else know that he is leaving? He is just simply asking for a "raise". He is not

I don't know how you come to that conclusion. How would the manager or anyone else know that he is leaving? He is just simply asking for a "raise". He is not going around telling everyone, hey "i am leaving, give me a raise or i am gone". Now that would be plain silly, lol. As I said, you look around to leverage yourself so that you know how much you are worth in the market. Lot of devs out there, working at companies for over 5+ years tend to underestimate how much they are worth, and they become a bargain for the company. They are simply too shy to ask for just how much they are worth and companies usually don't give raises to employees (besides the annual small pct increase) unless asked upon.