Most court cases are very important to the people involved and of little or no importance to others. So if I lose my job and sue my employer it' important to me and my employer but it really doesn't affect anyone else. My family may be interested in what happens but no principle of law or new government policy will come out of my wrongful dismissal case
But sometimes a court case has real implications for people who aren't directly involved. Usually the cases that have such an extended impact are about the validity of government actions; while they directly affect only the parties they have implications for others. For example, a claim that a specific Metis organization has certain aboriginal rights affects that organization directly but it has implications for aboriginal groups across the country.
In cases where the decision will have broad implications it is possible for groups or individuals who are impacted and have something useful to say to seek to intervene and join in the case before the Court.
In the Supreme Court of Canada case on euthanasia had people intervening to give their views; some supported the idea of legalized euthanasia while others were firmly opposed. What was important was not their position but the wealth of knowledge and insight they were able to give the Court.
Obviously intervening has an impact on the case – the more people involved the slower the case proceeds and costs are increased. For that reason Courts only allow intervention when the person seeking to intervene can show they are going to be affected and that they have something useful to say. The Court doesn't want random individuals to slow down ongoing cases so an intervenor has to show they are a legitimate and not just a malcontent. Similarly it is necessary to show the the intervenor can add something to the case that will not be brought forward by the parties already there.
Finally, so as to make sure there is a minimal disruption the intervenor is rarely allowed full participation in a case. Normally the intervenor can attend all court proceedings but cannot call evidence or examine witnesses – the intervenor is normally limited to making submissions on the law.
Of the Law Societies of Upper Canada and Nunavut