There are several distinct reasons that someone might wish to intervene in a proceeding: It is often said that the role of intervenors is to "assist" the court in making a just decision on the dispute at hand.
For example, the Quebec Secession Reference (a case in the Supreme Court of Canada) had one amicus curiae and several intervenors.
[2] In the context of judicial review, an interested party is 'any person (other than the claimant and defendant) who is directly affected by the claim'.
[3][4][5] For example, in Bell v Tavistock, the defendant (a specialist National Health Service clinic) offered GnRHa drug treatment to under-18 patients.
The complainant, who sought this treatment as a teenager, in 2020 posited that due to her age she was unable to give informed consent.
The applicant cannot sit on its rights; it must intervene as soon as it has reason to know that its interest may be adversely affected by the outcome of the pending litigation.
In addition, U.S. federal law does not allow the procedure of intervention to violate the requirements of diversity jurisdiction.
[7] While the Texas Rules of Civil Procedure require no judicial permission and impose no intervention deadline, common law dictates that a party may not intervene post-judgment unless the trial court first sets aside the judgment.