A fair question, and one we get a lot from school districts.
I will try to sum it up the best I can here, but I do encourage you to look at the materials from Virginia. They do a very good job summarizing things. At it's most basic level, a threat assessment team serves as a central repository for any and ALL reporting of concerning behavior related to a school district and it's entities. It is not a committee per say because it is the full time job of the members outside of a few exceptions (you hope to have district administration represented, teachers, etc.). Have a concern a student is going to hurt someone? It goes to them. Concern a student seems more withdrawn than normal? It goes to them. Student made some statement that made you go "huh, that was weird"? It goes to them. Student reports severe disruption in their home and it is impacting them? It goes to them. As it currently stands in most districts, there are very distinct silos which do not communicate with each other. The team at its basic level serves to remove any and all silos as it related to addressing concerning behavior within a district.
Now, what do they actually do. What they do not do is profiling. Profiling involves creating a profile, and then trying to match it to a population to try and see who might fit. This is the opposite. When a concern is reported (and we can have a lengthy discussion just about ways to increase reporting of concerning behavior), the threat assessment's team job is to 1.) Assess. Using the expertise of all the disciplines represented, you determine what the level of immediate, eventual, and periphery risk of harm is. Harm can take many forms obviously and is not limited to "is he going to shoot up the school tomorrow or no?" 2.) Mitigate. Once your assessment is complete, you take steps to mitigate. This is why having a multidisciplinary team with resources is essential, because your mitigation can range from calling the police to arrest someone who has already committed a crime, to connecting the students parents with employment services, to providing free of charge mental health counseling, to having the student complete check-ins with a staff member every morning, etc. The team identifies and implements the steps and resources necessary to address the concern. And the most vital piece is step 3, which is ongoing management. Some threat assessment cases never end. The team continues to track and manage cases as long as the individual is associated with the district. Ongoing assessments of levels of risk are conducted, and mitigation attempts are adjusted as needed. Many security approaches now pretend that once you kick someone off the island or show up at their door once, it is over. That is terribly false and ongoing management is essential because people swim back to the island.
Those other things you mentioned (i.e. identify schools most at risk of a shooting, harden targets with security, etc.) can also be completed by threat assessment teams if the district wishes. However, their primary function and purpose is to remove any confusion or trepidation about what to report and where to report it on the part of students, teachers, and community members, and then to perform behavioral assessment and ongoing management of individuals who presented the concerning behavior. It works. Kids do not wake up one day and decide to bring a gun to school. Every single school shooting without fail had a multitude of warning behaviors that were noticed, but nothing except maybe a brief law enforcement interaction with limited continue monitoring and management was done either because that was all that could be done given the system and resources, or nobody thought to do something else. When these early warning behaviors are identified, reported, mitigated, and managed, you stop people, like I said, at a hypothetical step 2 instead of them continuing to the hypothetical end point of a gun at school.