UI's paradigm people use is windows/desktop paradigm for a long time now, so maybe not necessary but they will often look to a place where window title should be just to make sure are they on the right track.
Also window title can contain additional and maybe important info like:
- what tab are you on (browser)
- what document are you on (word)
- what image are you editing (photoshop)
- how many new mails (email)
- title text can be animated so it can display alternating info (little * if application needs user attention)
You need minimize, maximize and close buttons somewhere so that bar where those buttons are located would be rather empty without window title. And also, when users have many windows opened on desktop they will rely on windows title to recognize window they search for.
There are exceptions of course, but are rare and used for special applications.
I don't know how easy is to implement window-title-barless window on OSX but on Windows I think you have to dive into Win32 API which is not praised by many.