Changing a club's annual picnic date and selling its building need not require the same level of agreement. A supermajority rule sets a threshold above one half for decisions the group wants to make harder. Two-thirds is a common example, but the actual fraction is a design choice or a requirement of the governing rules.
Such rules give the existing arrangement extra protection. A proposal supported by 60% can fail under a two-thirds threshold, leaving a 40% minority able to block it. The different voting requirements of the US Senate illustrate how thresholds can vary with the action being taken rather than applying uniformly to every decision.
Protection can become a bargaining resource
A housing cooperative might require broad agreement before taking on a large debt because the consequences outlast the current committee. That can encourage negotiation and revisions that ordinary majority voting would not require. It can also let a blocking faction demand concessions unrelated to the proposal.
The denominator must be stated. Two-thirds of ballots cast, members present, and total membership are three different requirements. Abstention can have very different effects under each. If the threshold makes adaptation nearly impossible, protecting the institution may leave it unable to repair its own rules. The protection is useful precisely because it is an obstacle; deciding which changes deserve that obstacle is the political choice.