As an outdoor recreation writer, I get this question often. I have tried many apps, and like you, previously used a Garmin Oregon 450T (still have it, in fact) and now use an Oregon 750T.

Simply put: Your 750T has a better antenna,  and better receiver, than your cell phone. It can also be "fine tuned" by adjusting the sampling rates to improve accuracy, something else your cell phone can't do.  From a safety stand point, your 750T will last FAR longer on a set of lithium batteries (or on a charge, if it has a rechargeable battery pack) than your cell phone will last on a charge, especially if you're actively tracking with a GPS app.