I'll give you two examples. One functional and the other OOP. Both programs aim to simulate driving given an input of 10 energy units to find the final output energy.

  #oop

 engine = Engine(10)
 car = Car(engine)
 car.drive() #result 8

  class Car:
    def __init__(self, engine):
     self.engine = engine

    def ignite(self):
     self.engine.energy =- 1

    def run(self):
     self.engine.energy =- 1

    def drive(self):
     self.ignite()
     self.run()
     return self.engine.energy

 class Engine:
  def __init__(self, energy):
   self.energy = energy


 # ignite not testable without engine
 # run not testable without engine
 # drive not testable without engine and a car
 # ignite, run, and drive are not modular cannot be used without engine. 
 # engine testable with any integer. 
 # Car useless without engine
 # engine useless without car




 #functional \
 def composeAnyFunctions(a,b):# returns function C from A and B. See illustration above. 
  return lambda x: a(b(x)) 


 def ignite(total_energy):
  return total_energy - 1

 def run(total_energy):
  return total_energy -1 

 drive = composeAnyFunctions(run, ignite)
 drive(10) #result 8

 # compose testable with any pair of functions
 # run testable with any integer
 # ignite testable with any integer
 # drive testable with any integer
 # all functions importable and reuseable with zero dependencies. 
 # input_energy -> ignite -> run -> output_energy

"I think the lack of reusability comes in object-oriented languages, not functional languages. Because the problem with object-oriented languages is they’ve got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle." - Joe Armstrong

you don't necessarily need the car or engine to simulate the energy output of driving.