lets both Shepard and Ryder pursue a romantic interest, or multiple, depending on how Shepard can switch love interests through the trilogy.
The optional romance that the player can choose gets the most focus, of course, but there are romances between NPCs in the world that Shepard can stumble upon while completing missions, and sometimes, Shepard's choices can impact the outcome of those relationships.
It's one of many reasons why it's rewarding to pay attention to details throughout the trilogy. Illium is a beautiful planet, and Shepard will likely visit it multiple times in, which is also where they can overhear an interesting conversation about an Asari theory.
While over a dozen missions and assignments require going to Illium, Shepard doesn't have to complete all of them in, since the game allows the player to make decisions, and that can include not recruiting characters, or even skipping their loyalty missions to accept the increased risk of death during the last mission.
In a standard run, however, Illium is a frequently visited planet. While on Illium, Shepard can come across Ereba and Charr, who are at a difficult point in their relationship.