The value of monads is that they fold side-effects into the return values.

  dirty languages: 

  input -> function_a -> output/input -> function_b -> output 
               ^                             ^
               |                             |
          side_effect_a                 side_effect_b
               |                             |
               v                             v
          lovecraftian_primordial_soup_of_global_state

  pure languages: 

  input -> function_a -> output/input -> function_b -> output 
               ^                             ^
               |                             |
          side_effect_a                 side_effect_b ------>
               |
               +-------------------------------------------->

If C++ were pure, the type-signatures would look like

  (output, side_effect_a) function_a(input);

  (output, side_effect_b) function_b(input);

The drawback is that the type-signature of function_b(function_a()) becomes complex. Now, function_b needs to accept and pass-on the upstream side-effects. To compose function_a and function_b, we need to convert the type-signature of function_b to

  (output, side_effect_b, side_effect_a) function_b(input, side_effect_a); 

Fortunately, ">>=" converts function_b under the hood. Which allows us to write

  function_a() >>= function_b() >>= function_c >>= function_d

and pretend that each ">>=" is just a ";" without wrestling with compound inputs and compound returns.