The Main Quests are a type of Quest in which The Journalist explores Snaktooth Island to find and meet the scattered expedition members and convince them to go back to Snaxburg.
Filbo explicitly asks the Journalist to do so, assuming the members have valuable information which might help in finding Lizbert. The Journalist explicitly needs to interview Lizbert as per demand from their boss Clumby or they might lose their job.Finishing the Main Quests and Interviews are necessary for completing Lizbert Goes Missing, which is both the full story as it is Lizbert's Main Quest.
Since the events of The Fight after Lizbert's disappearance, each member has left town either out of safety, mental health or to escape the consequences of their own actions. Filbo left in an attempt to find Lizbert. Though unsuccessful, he did find the Journalist, or rather, the Journalist found him.
The individual reasons for parting ways are elaborated on in the Interviews, which are held after their Main Quest and have returned to Snaxburg. The Main Quests do not mention these reasons and are not "solved" nor "concluded" upon their return. Instead, The Journalist is put on often odd tasks by the members in their own interest and benefit. The community remains comically unstable throughout the rest of the story.
While not explicitly stated in-game, the Trailer audio has Filbo speak of The Journalist as the replacement of Lizbert in the Bugsnax catching task and this being the way to get the Expedition Team back. While all Main Quest tasks involve the Bugsnak catching, none of the characters ever note the role of the Journalist in the regard on Lizbert.
After a set "wave" of members return, a Story Event becomes available that opens the possibility to reach new locations and get the next wave to return and so on. Every inner biome hosts two expedition members and every outer biome hosts one. Most members' quest involve giving the Journalist a new Tool that will aid them in catching Bugsnax.