After battle of the gods, Piccolo comes to a realisation (U.k. spelling) that if there are more threats like Beerus out there in space ready to destroy Earth than he needs to be at his peak and his regular training regiment just isn't working anymore because even if he is in the top 4 of the Z fighters at Fourth position, it's nothing compared to the level Goku and Vegeta have just claimed with God Ki on their sides bolstering their Ki to new peaks and with Gohan not training any more he thinks that there needs to be more than just Goku and Vegeta to defend Earth so he needs to find a way to get to that level of power which is where the ancestry of the nameks comes into play with Goku ITing with Piccolo to Namek where they are told by Moori that when Guru passed,

he did so by transferring the Ancient histories and legends of their people and the knowledge of Porunga and in these history/legends is where it is said that only when a Namek as increased to a specific plateau of power(which he has as his power isn't increasing) they would feel a block be in place(which he does feel now that he is aware of it) that is stopping the Namek from going to the next level of Ki incase of the Namek's corruption(SLUG) and only the Elder Namek could make a crack into that power that is behind that block and that Piccolo would need to Gradually release that divine Ki into their normal ki eventually only having Demigod Ki (which is what Goku and Vegeta have in ROF but is only fully activated in SSJ Blue)

which allows him to access the Great Namek form's natural strength( higher than Piccolo's normal strength) and the more complex thoughts that the God Ki activates when it is coursing through his body(specifically the brain). and after the year between BOG and ROF we see Piccolo go head to head with Golden Frieza in which Piccolo's eyes are a different colour or his pink forearms are glowing with Demigod Ki to signify his God form is at full power and thats when Beerus tells him to stop fighting as he wants to see Goku fight Frieza, he complies but blames the two of them if Frieza does anything really bad(...Blew up the Earth) and that when he threatens them that he has the strength and mindset to defeat him and if he doesn't have the strength than he soon will have as he has only unlocked a specific amount of the God Ki that is behind that wall(a percentage over 75%) so it's on them if anything happens as he was on Namek and he knows what happens when Frieza thinks he is going to lose.
Things to remember : Piccolo doesn't fight like Goku or Vegeta, he doesn't gradually release his power in response to his opponents like Goku does.