Defaults quietly shape behaviour because the easiest available option is often the one people keep.
A default is a decision made in advance on someone else’s behalf. In software, it might be a setting or a suggested option. In daily life, it can be the route or routine that requires the least effort. People can choose differently, but many will stay with what is already there.
That gives defaults more influence than they appear to have. A good default reduces work and helps people get a sensible result. A poor one can shape behaviour without much awareness. The details of how a choice is presented matter, even when every option is technically available.