Automatic charging and power switch between USB and LiPo battery
Compare the two schematics in detail at the relevant point: -



The top design uses a P channel MOSFET and, when the VBUS power is removed it will turn on that MOSFET and apply full battery voltage to the voltage regulator (U2).

The lower design uses a schottky diode in place of the MOSFET and so the battery voltage passed to U2 is somewhat lower (circa 0.25 volts to 0.3 volts) and so the circuit will fail to operate slightly earlier than the top design as the battery depletes.