Typically, experts are relied on by both sides to a dispute for opinions on severity of injury, degree of insanity, cause of failure in a machine or other device, and the like.
The tribunal itself, or the judge, can in some systems call their own experts to technically evaluate a certain fact or action, in order to provide the court with a complete knowledge on the fact/action it is judging. The expertise has the legal value of an acquisition of data. The results of these expertises are then compared to those by the experts of the parties.
The expert has usually heavy responsibilities, especially in case of penal trials and the expert's false witness is in most countries a severely punished crime.
In most systems, the trial (or the procedure) can be suspended in order to allow the experts to study the case and produce their results.