The charge of threat to commit a crime commonly accompanies assault and battery charges where there is some kind of heated exchange between the people involved. In other cases, a charge of threat to commit a crime is employed by the police where the police want to make an arrest, but no real offense has been committed.
There are several reasons police might include a charge of threat to commit a crime on top of other charges like assault and battery, but the most common reason is charge stacking: a technique by which police exercise their discretion to charge every single offense they can think of, rather than limiting their complaint to the charges that really fit the facts they allege. The primary reason for doing this is to signal the prosecutor and the court that the police consider a particular case to be more serious than the charges might otherwise indicate (i.e. to say 'this is not just another assault and battery case, there is more going on here.').
In any case, the charge of threat to commit a crime comes from General Laws Chapter 275, Section 4. Chapter 275 provides for specialized procedures to be used when charging a threat to commit a crime, but the courts have held that those procedures do not need to be followed. Personally, I have never found the reasoning in those cases to be persuasuve, and I have argued on several occasions that courts should not follow those decisions.
Additionally, the federal courts have held that the Massachusetts statute criminalizing threats to commit a crime could be read to go too far, and end up criminalizing speech which would otherwise be protected by the first amendment. The federal courts have therefore held that the statute must be read in such a way as to be limited to "fighting words" or "true threats." The upshot being that, although the police may consider a statement to be a threat, and although the Massachusetts statute may be ambiguous enough to cover it, there remains the question of whether those words can constitutionally be criminalized under the first amendment.
The charge of threat to commit a crime is, therefore, much more complicated than the title of the charge implies. If you, or anyone you know, has been charged with threat to commit a crime, contact my office immediately to discuss whether that charge could be dismissed before trial.
See: G.L. c. 275, sec. 4; Commonwealth v. Daly, 12 Mass. App. Ct. 338 (1981); Robinson v. Bradley, 300 F. Supp. 665 (1969)