The point is to express love and respect for someone who obviously means a great deal to them. It's not for Grandpa to see it and feel good. It's for everyone else to see and feel good. Also, saying "I hope you still remember us" might suggest that Grandpa is dealing with Alzeimer's.
If it "always aggravates" you to see others expressing their love and respect for their elders, especially their ancestors, then maybe Facebook is not for you.