What is this toggle u guys referring to?
May have an impact on your mobile data mileage. Apps like youtube may pick the "best quality" video playback your current bandwidth can support.
Lower bandwidth (i.e. in HSPA) may influence apps like these to go easy with how much data it eats. Also 2G/3G/4G runs on different networks. Good signal strength for a particular band is not representative that all other bands will be stable and strong.
i.e. if your office has weak 4G and decent 3G, you may want to prefer a 3G connection or else your phone would trip between 4G and 3G (more battery drain as your phone searches for networks actively etc...)
Also, sometimes there are carrier network issues with a specific band. i.e. 4G gives you 5/5 bars, you can't connect to the internet at all, 3G works however. As such, you may want to let the phone prefer 3G. vice versa.