This is a great chance to advance anything you want to have happen in the campaign. For example, if you want to develop the character background further, have them belong to some organization (city guard, magical college) you want to establish. You could have characters make something up if you want them involved in the narrative. They might claim to be professional explorers themselves- how will anyone check?
For your specific example, expendable does seem to make sense, if you just want a justification. They might send anyone who will go, especially if they only pay upon return. They might want to clear the area of rowdy adventuring riffraff as well. This can get the party together, if you need to, because they are one of the randomly thrown together teams. If it's too expensive to make sending everyone make sense, there could be one giant expedition that will inevitably be separated.
If the characters are all 1st level, there might be a tradition of a coming-of-age journey and the characters all come of age at the same time. This year the journey is sent to the new continent because whoever decides where it goes wants the continent explored.
If your group has played a lot, the getting hired thing is so cliched that you can even do it comically - the nobleman walks past dozens of eager, heroic subjects, approaches the party, peers at them intently for a while, then declares "I like the cut of your jib!". After a lively whispered debate with part of his retinue, he makes the players an offer, to their obvious disgust. (Better if he is offering resources that basically make no sense - this lets the players know that the campaign is more light hearted and they don't need to spend hours checking out their employer's motive.) If this is how the party comes together, they can be the noble's hand picked crew (maybe throw in a competent NPC or two who discover other obligations when they realize the rest of the crew is the PCs).
If the characters are going to frequently be singled out for no good reason you can do some variant of the "chosen one" trope. That can consolidate lots of contrived plot devices into one major contrived plot device, at the expense of being really lame unless who they are chosen by makes up for it.