What is the purpose of MOSFETs in this schematic?
Q1, Q2 and Q5 are under host control and determine if battery is charging, and if 'System' is on or not. Q1 and Q2 select whether the battery or the adapter is sending power to U1. Q5 works with Q2 and both are 'ON' when the system is battery powered. U1 can be pulsed 'ON' (Q2 is 'ON') to maintain charge under load. This circuit can both charge the battery and supply system power if required. Todays smart phones can both charge and play videos or surf the net at the same time. Older cell phones had to charge first before you could use the phone as a phone, or send messages, etc. The newer charger IC's (U1) can handle the extra load of doing both task at the same time. If the phone is just charging, Q2 and Q5 will be 'OFF'. The user turning on the phone will cause Q5 and Q2 to come on in the proper order to supply system power. If the adapter is not plugged in or the battery is at full charge then Q1 is 'OFF'