The fact that missions in Helldivers 2 have a time limit means that new players will usually make a beeline straight for the main objectives, then extract as soon as possible. Doing so causes you to miss Areas of Interest, hidden locations that contain weapons, currency, and supplies. Areas of Interest are usually off the beaten path, so if you’re part of a team it can help to spread out while traveling between objectives to maximize your chances of finding one.
Most Helldivers 2 maps also have at least one hidden Secondary Objective. Finding and completing it will earn more XP and Requisition Slips. The same goes for enemy outposts, shown in red on the minimap, but those are always visible so you can plan your routes to destroy or avoid them as the situation dictates.