It provides structure to the interview process. Instead of asking how a candidate would behave in a situation, the interviewer asks how did the candidate behave. Interviewer can question and probe candidate responses. Candidates usually have to talk more and provide more details. Interviewer can target specific information more easily.
Interviewer is more interactive with candidate.
Candidate may have less control over telling prepared stories. Interviewer more likely to take notes. Think of relevant situations that reflect favorable actions. Prepare short descriptions of each situation but give details if asked. Include the outcome or result of action.