As is mentioned on the wikia site,
Guardian (神様, Kami-sama; lit. "God") is the title given to the individual protectors of planets in the Dragon Ball series.
Despite their names, the Guardians' role is closer to that of an overseer.
As the Guardian of Earth (I will be using GE as an abbreviation), Kami or Dende have to protect the human beings from evil. A few examples being
- Kami training Goku during the Piccolo Jr. Saga.
- Kami training the other Z Warriors during the Saiyan Saga.
Kami and Mr. Popo spreading the Sacred Water throughout the entire world using the Seven Air Currents located inside Kami's Lookout in order to cure the Earthlings from the effects of Garlic Jr.'s Black Water Mist.
To expand a bit more on their job,
Guardians have an indirect reporting relationship to one of the Kais (in the case of the Guardian of Earth, this is King Kai).
Another way of looking at this is that Guardians are more supervisors than they are managers.
They have many of the powers associated with Gods, including the ability to see and sense events around the world, to travel to and guide souls to Other World, and to be a general protector of the people.
This concludes the tasks of the GE.
Now to answer your other questions,
- The GE doesn't necessarily have to be from Namek. The reference being that
- Kami was the first Namekian GE.
- Kami asks Goku to replace him as the GE after the World Martial Arts Tournament in Piccolo Jr. Saga.
- As the GE doesn't have to be Namekian, they don't have to create the dragon balls.
- They do grant human wishes if they are appropriate. An example being when he gave a new life to Shenron on Goku's demand after he was destroyed by King Piccolo.
I believe this answers all your questions...