Hello! This is a great question!
Ikalgo's friendship with Killua is vital both to Killua's development and also to Chimera Ant arc as a whole! I can explain that significance.
Throughout much of the series, Killua tags along with Gon and primarily focuses on him. He befriends other people, certainly, but he generally does this through Gon--it's not like he's going out of his way to make these friends on his own.
In Chimera Ant Arc, the connection between Gon and Killua gets somewhat disrupted by the events that happen, and Gon's focus shifts primarily to saving Kite and his revenge on Pitou, leaving Killua to worry and feel somewhat left behind and helpless (partly as a result of the the Palm situation as well).
When Killua splits off from Gon to try and save civilians, he meets and a battles an enemy at the time, Ikalgo.
During their fight, Killua gives Ikalgo a choice: Either sell out his friends by revealing their nen powers, or die. Ikalgo chooses to die rather than betraying his friends, and Killua is able to relate to this loyalty. He spare's Ikalgo's life and tells Ikalgo that if they'd met another way, maybe they could have been friends.
Killua's kindness in empathizing with Ikalgo and sparing him is the only thing that saves his life later when he's bleeding out from the needlefish darts. Had Killua not extended that mercy and said those kind words to Ikalgo, Ikalgo would not have saved him, and he would have died.
It's interesting/neat how the scene right after Ikalgo saving Killua from the needlefish involves Gon having his own encounter with another friendly Chimera Ant, and the tone of their conversation is quite a bit different:
Contrast is so important in Chimera Ant Arc!
Anyway, when Killua shows Ikalgo mercy, makes it clear that they're friends now, and invites him to come along and be part of the human side of the war, he essentially becomes to Ikalgo what Gon has been to him: Someone who represents light, and who represents a path forward to a better life. In this arc, Killua worries that he doesn't deserve a place at Gon's side, but at the same time here he extends the same kind of light and hope that Gon gave him to someone else.
As for the scene where Killua breaks off away from Gon in the Palace Invasion in order to protect Ikalgo, this continues to show Killua's world expanding outside of Gon. Notice how in this page, it clearly shows Killua leaving Gon behind--they have their backs to each other, which is a frequent image in this arc. His mission was to protect Gon and make sure Gon could complete his revenge against Pitou, so deviating from that mission is both shocking to him, and potentially dangerous, but it shows that he's starting to protect and care for others as well as Gon.
Now, I want to be clear that I see this as a positive step for Killua. At this point in the series, Killua had made Gon his entire reason for living, essentially, and him going outside of that and making other friends and protecting and caring about them is a vital step for his character. It's showing that he's becoming a more expansive, mature person who can care for multiple people (or ants) at once rather than centering his entire existence and sense of self around one person. He made this action as quickly and efficiently as possible so he could get back to Gon's side, but he did go out of his way and leave Gon behind momentarily, and even he is surprised by this.
And while obviously I adore his nearly single-minded love and devotion for Gon, with what happens between them in this arc he needs to have others in his life besides Gon, and it's part of his transition into also being the light for Alluka and Nanika. It's a result of Killua's own innate kindness and empathy and the choice he made to leave his previous life behind, but also, he was able to reach this point partly because Gon showed him the love and encouragement he needed to become the person he is.
Now, as for Ikalgo's role in the arc as a whole: He's part of a chain of kindness and mercy that ends up changing the entire outcome of the Chimera Ant arc. This post puts it incredibly well. The chain of kindness started at the beginning of the series, with Gon befriending and rescuing Killua in spite of his background, which then eventually leads to Killua sparing Ikalgo (who then saves Killua right back), and then Ikalgo sparing Welfin, and then Welfin giving Meruem Komugi's name so that they got to be together in the end. Ikalgo shows the consequences of Killua's kindness, which are also the consequences of Gon's kindness, in part.
A major theme of the series and of Chimera Ant arc is that love and kindness are transformative, and Ikalgo has such an important role in demonstrating that theme.
