WARNING: The following contains spoilers for Sailor Moon Eternal: The Movie, now available on Netflix.
The first three seasons of Sailor Moon Crystal introduced two sets of Sailor Guardians: the four assigned to protect Princess Serenity known as the Inner Guardians, and the four assigned to protect the solar system from alien threats known as the Outer Guardians.
The Inner Guardians are comprised of Sailors Mars (Rei Hino), Venus (Minako Aino), Mercury (Ami Mizuno) and Jupiter (Makoto Kino). The Outer Guardians include Sailors Uranus (Haruka Tenoh), Neptune (Michiru Kaioh), Saturn (Hotaru Tomoe) and Pluto (Setsuna Meioh). Princess Serenity herself reincarnated as Usagi Tsukino, the Sailor Guardian who often leads the Inner Guardians as Sailor Moon. Throughout Sailor Moon Eternal, each of the Inner Guardians finds themselves questioning who they are out of costume.
Ami thinks she wants to be a doctor when she grows up. Deep down, however, she is quite lonely and yearns for meaningful relationships after being impacted by her parents' divorce earlier in her life. Rei similarly thinks she wants to take over the Hikawa Shrine as the head priestess once her grandfather passes away. However, she feels abandoned by her father, who is a politician, while her mother is deceased. Makoto expresses a desire to marry and become a business owner, and Minako expresses a desire to become an idol -- but both girls suffer from feelings of inadequacy.
When the Amazoness Quartet from the Dead Moon Circus exploit each of these insecurities in the Inner Guardians, all four are reminded of how far they've come since awakening as Sailor Guardians. They don't just exist to be bodyguards to Princess Serenity and her Prince Endymion, nor do their entire lives revolve around their responsibilities as Guardians. Since meeting Usagi, they've become the best of friends, often enjoying normal outings together that don't necessarily involve saving the world. Even if they don't end up pursuing their individual life goals, they are fine with that because they have something much more valuable to them: friendship and family.
This realization releases the Inner Guardians from the stranglehold of the Dead Moon Circus and facilitates their power upgrades into their super forms. But this is only true for the Inner Guardians. The Outer Guardians are a different story.
When the Outer Guardians appear in the second half of Sailor Moon Eternal, they do not find themselves in a better place. As soldiers who only get summoned during crises involving alien threats, they're rarely needed by Sailor Moon or the Inner Guardians as they can defeat most Earthly threats on their own. As the original loners of the solar system, the Outer Guardians find comfort in each other's company, even when they go about their usual lives. For Haruka, this means partaking in car races. Michiru gives concerts as a violinist while Setsuna works as a scientific researcher in the field of theoretical physics.
As the younger and more destructive member of the Outer Guardians, Hotaru spends most of her reincarnated existence as an infant and is only awakened when a new alien threat arrives on Earth. Despite this, however, she manages to bring the Outer Guardians together into an ersatz family to care for her. This gives them another reason for being together that has nothing to do with battle.
When Saturn is newly re-awakened to face the threat of Queen Nehelenia and the Dead Moon Circus, she reminds the Outers of their responsibilities as Guardians -- but also that they don't have to wait for a fight that requires their unique power set. Just because their original function is to protect the solar system from outside threats, that doesn't mean they can't protect the princess they also serve, along with the Earth that is now also their home.
This is what prompts the Outer Guardians to reunite with the Inner Guardians in their fight against the Dead Moon Circus. They are ultimately united in the same goal to protect, which facilitates Sailor Saturn to upgrade their powers into their super forms. When they all meet at the same place, it is only through their combined powers that they are able to provide Sailor Moon, Sailor Chibi Moon, and Tuxedo Mask with the tools needed to put an end to Queen Nehelenia's terror in Sailor Moon Eternal.