I would think that especially for fitness purposes, the watch will not require a phone to do things. You should still be able to access all the stock fitness apps to see that data. Imagine you are running and you can't get an armband for your iPhone 6 Plus because it won't fit on your arms, but you really want to listen to music. So you use bluetooth headphones with your Watch. You don't need to take your iPhone with you because it can still play music stored on it, it can still track fitness. It just can't do the things required through iPhone relay. You won't get notifications from apps like messages or phone or third party apps. (unless perhaps you are running on a treadmill and your phone is close enough for bluetooth to connect to the watch).
It does need an iPhone to work completely, but it will work without it. It will always work as a watch.