An egress window is a way out in a fire and a way in for a firefighter wearing gear. That is why the requirement exists and why it is written in terms of a minimum clear opening, a maximum sill height above the floor, and a window well big enough to climb out into if the window is below grade.
The exact dimensions come from the building code your jurisdiction has adopted, and they are enforced locally, so the authoritative answer for any specific house comes from the building official for that city or county, not from a contractor and not from an article. Confirm before you build, not after.
The practical consequence people care about is resale. A finished basement room without compliant egress is not a bedroom, whatever it is used for. An appraiser or agent will not count it, which changes how the whole house is described.
The work itself involves cutting the foundation opening, setting the window and the well, and finishing the surround. It is heavy, dusty work that happens before finish work. Retrofitting one into a finished basement means undoing part of the finish first.
If you are planning a basement finish, handle egress early. It is far cheaper as part of the plan than as a correction.