When account A has logged into computer A, and then account A starts to log in to computer B, account A in computer A will automatically be logged out. May I know what are the possible reasons in why this feature should be implemented? How does it make it more secure?
From the perspective of a developer who recently has to implement such a feature, another possibility is to prevent the users shooting themselves in the feet.
Many custom/internal applications have very specific workflows and store a lot of state in the session. If the user abandoned some work halfway on one computer, finishes it somewhere else and return to the original computer, he/she may get confused about the unifinished work. If the user incorrectly chose to continue, then the original results may be overwritten / duplicated.
It is of course possible to improve the code to make this less likely, but for an internal app, the most cost effective way is to prevent concurrent logins.
The most common case is that the login is used as a workplace accounting token, i.e. "check how much time you're working on PC". The more logical reason, but it's not easy to see from a first sight, is that you simply physically can't be on the two geografically isolated places at the same time.