In the secular court system, it is accepted that as part of the process of accepting witnesses, they are interrogated. The purpose of this interrogation is to ascertain to the degree possible that the witnesses are not lying and that they indeed remember in sufficient detail that about which they are testifying. Fundamentally, there is interrogation according to Torah law as well.
The mishna says: "They would check the witnesses with seven chakirot (a certain type of questions): During which sabbatical cycle did [the event] take place? During which year? Which month? What day of the month? What day [of the week]? At what time? In what place? Do you recognize him? If he was worshipping an idol, which was it, and how did he worship it?" In general, we can say that the interrogation deals with the timing of the event, the place, and certain crucial parameters about the event.
The mishna also discusses a secondary set of questions called bedikot. These are more detailed descriptions of the event, which are less fundamental to the legal issue at hand. The mishna says that the more bedikot that are asked, the better. The difference between chakirot and bedikot is that regarding the former, if a witness says that he is not sure of the answer to the question, his testimony is inadmissible, whereas regarding the latter, a witness can say he does not remember. However, even regarding bedikot, if the witnesses contradict each other, their testimony is thrown out. The type of question that would be included in bedikot is on details such as a description of the murderer’s clothing and the appearance of the ground upon which the murder took place.
There are three ways to explain the need for chakirot: 1) It makes it possible to expose the witnesses as eidim zomemin, witnesses who are confirmed to have been at a different place from the one where they say they saw the event at the time they said it transpired. 2) It clarifies the testimony, making it more likely that the witness remembers the story accurately and that he is not either mistaken or overly vague. 3) It gives the opportunity to find contradictions between the set of testimonies of the two witnesses.
Based on the first reason (the possibility of being zomemin), we understand why the absolute need to answer the questions applies specifically to chakirot. Only if the witness has to commit to the exact time and place is it possible that someone will be able to say that he was elsewhere at the time in question (Sanhedrin 41b). Rashi (to Sanhedrin 40b) calls these questions the chakirot of hazama. This is distinguished from other important chakirot that are required for other reasons, e.g., that the offense was committed in a manner that there would be capital punishment.