Sailor Moon fans are well aware that Queen Serenity and Princess Serenity are powerful beings, so why do they need the Sailor Senshi to protect them? Each member is a princess of her respective planet, so why do they need to put their lives on the line for the royal family of Earth's moon? Shouldn't they also have their own elite guards to protect them? The mundane answer is that Naoko Takeuchi wrote each arc like it would be the last, so every time the manga was renewed for a new arc, she would have to come up with some new revelation, thus creating some plot holes within the story. But let's see if we can't find an in-universe explanation.
Technically the Sailor Senshi aren't tasked with protecting the Moon Royal Family alone. They're meant to protect the Solar System as a whole. Each system has its own set of guardians, as seen in the Stars arc with the Sailor Stars and Galaxia. During the Silver Millennium, there was little trouble within the Solar System, so there wasn't much need for the Senshi to fight. Instead, they became tasked with teaching and guarding the young Princess Serenity, who was much less mature and preferred pranks and playing, much like Chibi-Usa later would. It makes sense that someone who would go off wandering to Earth and playing would need to be looked after by someone more mature.
The reason the Senshi protect the Moon Royal Family also stems from the fact that the Silver Crystal is extremely powerful, much more so than the crystals of the surrounding planets. The Crystal has been used to reverse the destruction, and in Neo-Tokyo, it keeps the citizens young and healthy. No such power has been shown coming from the planetary crystals. It's unclear if the planetary crystals hold any sort of powers other than allowing their respective Senshi to transform and use powerful attacks. Presumably, the Silver Crystal is significantly more powerful than any of the others based on evidence in the series. The only crystal that may come close to that amount of power is the Golden Crystal from Endymion's kingdom on Earth.
Since each Senshi is a princess, it makes sense they would be the ones to use the planetary crystals to protect their homes. There are historic records of royalty participating and dying in battle, so it's not a far stretch to think that this would extend to fictional space people. But what about their parents? It's also been stated that Sailor Pluto is the daughter of the Greek God of Time, Chronos. Queen Serenity has also stated that she has been worshipped by the people of Earth as the Goddess Selene. Taking this into account, we can assume that the other Sailor Senshi are also children of gods and goddesses.
As seen in mythology, the gods don't like handling things themselves and would rather get others to do the work for them. Considering that Chronos let Queen Serenity lock his daughter away to guard the Space-Time Door alone while she was still a child, it's not hard to believe the other gods would let their children guard the galaxy.
So while the Sailor Senshi do protect Princess Serenity even though they themselves are princesses, it's most likely to protect the galaxy as a whole. The Silver Crystal is more powerful than the planetary crystals, and since it belongs to the Moon Royal Family, it makes sense that they would need to protect them as well. It's also probably because they were tasked to do so by their parents, potentially the gods of Greek and Roman mythology. Or, if you want to be boring, just accept that Takeuchi just didn't provide a reasonable in-universe explanation.
