Trying to understand how intent(in) affects pointers and targets

Help
1

Here is my example code :
main program :

program main
  use,intrinsic :: iso_fortran_env,only: int32
  use test_pointers
  implicit none

  integer(kind=int32),target :: number1,number2

  number1 = 10;number2 = 30

  print "(a,1x,i0)", "The sum is:",sum_of_two(number1, number2)
  print "(a,1x,i0)", "The value of number 1 is:",number1

end program main

The module test_pointers:

module test_pointers
  use,intrinsic :: iso_fortran_env,only: int32
  implicit none
  private

  public :: sum_of_two

contains

  function sum_of_two(a,b) result(two_numbers_added)
    integer(kind=int32),pointer,intent(in) :: a,b
    integer(kind=int32) :: two_numbers_added

    two_numbers_added = a+b
    a = a+1_int32

  end function sum_of_two

end module test_pointers

Compiled with :

gfortran -O3 test_module.f90 main.f90 -o test_pointer

Output:

The sum is: 40              
The value of number 1 is: 11

What is the purpose of intent(in) here ?
Why does it not prevent the function from changing the value of the target ?

However if I remove the pointer and the target attribute, the compiler throws an error at compile time.

test_module.f90:16:4:                                                                       
                                                                                            
   16 |     a = a+1_int32                                                                   
      |    1                                                                                
Error: Dummy argument ‘a’ with INTENT(IN) in variable definition context (assignment) at (1)
3

Thank you sir !