Why would any person approach the patent office for patent opposition?

To answer this question, one should first understand two types of patent opposition provided in The Indian patent act 1970, which is post grant opposition and pre-grant opposition. The grounds for the post grant opposition and the pre-grant opposition remains the same except that the post grant opposition is filed when the patent is granted and the pre-grant opposition is filed during the prosecution stage of the patent application. The patent opposition can be filed by any person interest.
The grounds for the post grant opposition and the pre grant opposition are:
Anticipation by prior publication
Anticipation by prior date, Prior claiming in India
Prior public knowledge or public use in India
Obviousness and lack of inventive step
Non patentable subject matter
Insufficiency of description of the invention
Non-disclosure of information as per the requirement or providing materially false information by an applicant
Patent application not filed within 12 months of filing the first application in a convention country
Nondisclosure/ wrong mention of source of biological material
Invention anticipated with regard to traditional knowledge of any community, anywhere in the world.
The patentee or the applicant wrongfully obtained the invention from the opponent or a person from whom the opponent derives title.

Case study:

An inventor approached us, claiming that an organization had file his invention. When we did our research, we found out that the inventor was associated with the organization for few years and during that period of association, the organization had promised the inventor a remuneration for assigning the invention to the organization due to some misunderstanding between the inventor and the organization both the parties ended their association with each other but the organization filed for the patent for the invention with the inventor’s consent. Once the patent application was published the inventor came to know that his invention was filed by the organization. We filed the pre grant opposition on the grounds on wrongful inventorship and we were able to get justice to the inventor.