Most airlines open and close their windows. JetBlue advertises it, and only accepts applications during the open period. Places like American accept applications all the time, but only rack and stack the active ones when their window is open. So the window opens and closes, but it's transparent to the applicants. (Which is why you should update weekly at a place like AA.)

The reason for windows has to do with employment law compliance. If you leave it open continuously, then it is impossible to prove compliance with hiring practices.