The short answer is that Hot Spots (and the mission's Objective) are Risk/Reward locations. Typically centered on a scenic item, (a radio tower, an abandoned vehicle, a stack of crates...) they contain valuable salvage and occasionally Anomalies/Artifacts. Getting close also causes Zone Hostiles to spawn who will attack the nearest player models in LOS.
These locations/the spawning can also be triggered by tossing a bolt into that reaction radius. (same rules as throwing a grenade) This is handy if your opponent happens to be in the open as the hostiles will spawn and attack them instead of you. (assuming you were in cover)