To check if you are just a mercenary who works for whoever pays more.
To know your experience that made you leave the previous job.
Insights:
Be ready with a convincing answer. The interviewer is trying to judge whether to hire you or not, if you have left your previous job after a very short stint.
Have a list of reasons which do not say bad things about your previous employer.
If you have had disagreement with your previous employer, do not wash dirty laundry in public.
Do not give a flippant reply. It shows that you are not a responsible person and do not take your job seriously.
Example Answers:
I had to leave in a very short period due to some family problems and continuing my job was not possible. Since I had not completed my probation, getting leave was difficult, when my family needed me the most. But I assure you that nothing of this sort will happen again now since the problem has been sorted out.
I had few disagreements with my manager in the earlier employment, and as I could not make compromise with my core believes, I thought it would be a good idea to seek out like-minded people who can foster those believes.
There was a gap between what I do with my life and what the organization needed, so rather than having a disturbance in my personal life as well as work envirnment, I decided to approach people with same vision as mine.
Anti Pattern:
Do not say left because you did not like the work environment or the project that you were assigned. It may indicate that you are not adaptable.
Do not say that you did not like working at the earlier organization and hence left the job.
Do not elaborate and dwell on the problem you might have faced earlier.
Do not portray yourself as a victim.
Interviewer's Interpretation:
The interviewer may appreciate you honesty and forthrightness as long as you can justify what you core beliefs are.
The interviewer will understand that you have the courage to take strong decision, but not out of malice.
That you are a sincere, thoughtful and responsible person who is capable of taking the right decisions,