Grus doesn't have a specific purpose in the story that we can pinpoint.
For Coby, we can assume for him to do what he promised - stop Luffy.
Can he realistically accomplish that purpose? Sure but he wont.
The narrative requires for Coby to fail in stopping Luffy because Luffy needs to succeed.
How would Coby suddenly be able to stop Luffy? Just give him anti-Luffy power.
Which power is that? CoC burst that negates DF abilities. Can Coby obtain that? Easily.
As soon as Coby would obtain DF-negating haki burst, he would even the playing field with Luffy.
While Luffy has more experience in actual fights, Coby is a trained soldier, which Luffy never was.
Remove DF from Luffy and Coby might actually have the upper hand, assuming he has aCoC.
You would probably come to the conclusion that such Coby is Admiral level. Is he though?
Perspective is everything. When you pair him with a DF merchant like Luffy or Blackbeard, Coby can look good.
But pair him against Zoro, Mihawk or Shanks and you will still see Coby as a toddler.