What is the purpose of making dangerous threats (bomb, poisoning, etc.) rather than simply going through with the attack?

I raise this question in response to this recent sad news.

We frequently hear of bomb threats, terrorist threats, poisoning threats, etc. What is the purpose of these threats? If truly intending to cause the harm listed in the threat, wouldn’t it be more effective to go through with the violence without warning anyone?

Sending out a “threat” in advance, in this case specifically, could likely lead to the bars closing temporarily or patrons refusing to go to the bar, thus, avoiding the poisoning. Similarly, other threats cause people to take measures in order to avoid the harm posed.

If you really want to harm people, I don’t understand why you would warn them in advance. The only purpose of threats seems to be to scare people; it doesn’t seem likely that a person intending to carry out a threat would issue the threat to begin with. So then, why believe them?

I am baffled by this concept of threats. Please enlighten me.