There are two main scenarios where you would use a mini implant.
If your jawbone presents some technical challenges that do not allow for standard implants. This could be because the jawbone is too slim to support a normal implant, or there are other teeth/ nerves in the area that mean there is less space for the implant. For instance, if the two adjacent teeth have roots that are close together, a mini implant may be the only option.
The other scenario where they are commonly used is to anchor a denture to the lower jaw. Because the lower jaw is the strongest bone in the human body, mini implants are more likely to succeed in this bone than the upper jaw.