You totally can use a LiPo without a MOSFET.
The reason you might choose a simple power regulating Metal Oxide Semiconductor Field Effect Transistor is to improve trigger response and prevent damage to the trigger switch contacts. It works by replacing one trigger contact wire with a control wire (blue below but usually very thin and black) and connecting the power wire directly to the motor.
When the trigger is pulled current flows to the MOSFET through the control wire which causes it to switch on and a higher current flows to the motor, thus the main power no longer flows across the trigger contacts. This eliminates the problem of micro-arcing causing damage to the contact surfaces, which increases their electrical resistance.
Compared to the rest of the circuit, the trigger is a high resistance component anyway and the more the resistance, the more power is consumed as heat rather than available to the motor. The main issue however is the time it takes for the motor to begin spinning from the moment the trigger is pulled, right? Trigger response. The motor needs to draw a much higher spike of current to get it spinning from the stopped position than it uses to keep it spinning. The battery supplies power as the result of a chemical reaction, which must start from nothing and jump to a momentary high burst as quickly as possible, but must take
some time no matter how good the battery is. When the power simply flows across the trigger switch, it begins as high resistance micro-arcs and then the resistance reduces as more surface area comes into contact, so that's a bit more time between trigger pull and motor starting to spin as the power available to it ramps up.
With a MOSFET it doesn't matter what the resistance across the trigger switch is because the power needed to flip the transistor to 'on' is negligable, so whatever power the battery can deliver is available to the motor pretty much instantaneously. This is where LiPo and LiFe batteries come in, because they are capable of a higher power output than NiMh anyway, but crucially their chemical reaction gets going faster too. Thus to really get the benefit of a modern lithium battery, fitting a MOSFET is the way forward.
