Why the thermistor needs a power transistor?

Hi, I'm just a noob in electronics so maybe this is a stupid question but I'm trying to learn so I hope someone will ask me
I would like to know what's the power transistor for the thermistor is used for. I think it's used to switch it on/off but I'm not sure.
Also does the thermistor need a lot of current? I saw the used MOSFETS can source a lot of it.

I think you are confusing thermistors with power resistors used for heating of heat bed and hot ends. MOSFETS are used for heaters which need more juice. MOSFETS are turned on/off with Arduino's D8, D9 and D10 digital outputs.
On the other hand, thermistors as temperature transducers are connected to Vcc trough R7, R1 and R11 resistors located on RAMPS board. The voltage drop across each thermistor is sensed by Arduinos analog inputs. This data is used for temperature regulation i.e turning on and off appropriate heater resistors.

Ops, I misinterpreted this part of the features list in the wiki:

- 3 mosfets for heater / fan outputs and 3 thermistor circuits

Now it's all clear, thank you!