There are two main reasons why an interrogator might want to offer rationalizations to a suspect during an interrogation.
First, an interrogator might do this to try to build a rapport with the suspect. By offering the suspect rationalizations, the interrogator will make the subject feel that he or she is on the suspect’s side. The interrogator will, in effect, be saying something like “I understand why you did what you did. It makes perfect sense.” By saying this, an investigator gets on the “good side” of the suspect.
Another thing that an interrogator might be doing is trying to elicit admissions from the suspect. By giving the suspect a rationalization for why they committed a crime, the interrogator is making it easier for them to admit what they have done. The suspect is, again, being told that what they did was understandable and was not some sort of a monstrous deed. This will make them feel less guilty and more likely to admit to things that they have done.
Thus, an interrogator might provide rationalizations to get in the suspects confidence, to get them to admit what they have done, or both.