Because not every homicide (killing of one by another) meets all of the elements of 1st degree murder a prosecutor MUST prove in order to sustain a conviction. 1st Degree murder has the highest threshold for proof and it usually has the most severe penalty as a result. Rather than risk everything on one big charge, a prosecutor may also charge the Accused with 2nd Degree murder--usually known as voluntary homicide--in case the prosecutor cannot meet his/her burden of proof for the 1st degree murder conviction. In criminal court, the burden of proof is beyond a reasonable doubt, and in some homicides charged as 1st degree murders, the facts may not support all the elements for murder.
In trying to prove the crime of murder took place, a prosecutor has to show that the homicide took place and that the Accused acted with the intent to kill this person. Intent gets into the psychological motivations of the Accused, and in some cases it might be difficult to clearly demonstrate the Accused possessed the intent to kill the victim. The prosecutor also has to show premeditation, or thought/planning the killing, which again goes towards defining the deliberateness elements of 1st degree murder.
How does this play out? Let's say Driver A cuts off Driver B in traffic, causing that Driver B to slam on his breaks and swerve to avoid another car. Driver B has a loaded 357 magnum revolver in his car that he usually keeps unloaded, but two days ago he went to the gun range to practice and left the gun loaded. Driver B grabs the gun out of the glove compartment, intending to point it at Driver A to scare Driver A and let him know his reckless driving is not appreciated, believing the gun is not loaded. Driver A sees Driver B pointing the gun at him and dares him to pull the trigger, which Driver B does and the gun fires, killing Driver A.
Driver B should be charged with 2nd degree murder because he didn't have the premeditation and intent to kill Driver A. He didn't know his weapon was loaded, didn't know Driver A before that day, and only wanted to scare Driver A and not harm him.
Contrast that scenario with this:
Everyday Driver B partially blocks Driver A's driveway with his huge Cadillac SUV. When Driver B asked Driver A to please move his vehicle, Driver B cursed out Driver A and said "he'll park where he darn well pleases, you loser!" Driver B shoves Driver A, Driver A shoves him back and they have a brief fight which ends when Driver B gives Driver A a black eye. Driver A never calls the police and doesn't speak to Driver B for several days. Driver A is furious about losing the fight and how Driver B disrespected him. His wife tells him to call the police or let it go but Driver A goes to his parent's house and takes one of his father's handguns--a 357 magnum revolver that holds five bullets. He puts in all five and waits for a day where Driver B blocks his driveway again. Several days go by before Driver B does it again. Driver A goes out to confront him, and Driver A says, "oh, back for more?" and takes a step towards Driver A. Driver A pulls his gun and fires 4 times, hitting Driver B three times in upper body. Driver B falls to ground, in great pain but still alive, and apologizes to Driver A and begs him to call an ambulance. Driver A walks over to Driver B and fires the last bullet into Driver B's back and walks back into his home, then calls 911.
This scenario fits 1st Degree murder because there is intent (fires 4 times, pausing between the last two final shots) and premeditation (he stewed over the fight for over a week, went to his dad's house to get the gun). He also avoided handling the issue like a citizen by calling the police like his wife suggested. Given these facts, a prosecutor would feel pretty good about bringing a 1st degree murder case here.
