Document the purpose of posting this on drupal.org?

What is the benefit of having it on drupal.org as opposed to just leaving it on github?

Usually when a project is mirrored between the 2 sites there is an explicit reason for mirroring it. Otherwise it should be left in the original place desired by the original author.

You still didn't answer the question of what you hope to achieve by moving it here.

Did you discuss having it on drupal.org only with the original author, GiorgosK?

For what it's worth, I did a diff of the src subsdirectories of the first commit here and the last commit on github and it's basically code style kind of changes.

diff -r simple_lexer_parser/src/ math_field/src/
diff -r simple_lexer_parser/src/Calculator.php math_field/src/Calculator.php
3c3
< namespace Drupal\simple_lexer_parser;
---
> namespace Drupal\math_field;
8c8
<  * @package Drupal\simple_lexer_parser
---
>  * @package Drupal\math_field
13c13
<    * Lexical analysis on a given arithmetic expression
---
>    * Lexical analysis on a given arithmetic expression.
16c16
<    *   an arithmetic expression in infix notation
---
>    *   An arithmetic expression in infix notation.
19c19
<    *   The result of the expression in postfix notation
---
>    *   The result of the expression in postfix notation.
22,27c22,26
<    *   on following occasions
<    *   Empty expression
<    *   Invalid character
<    *   Mismatched parentheses
<    *   Unexpected token
<    *
---
>    *   On following occasions,
>    *   Empty expression,
>    *   Invalid character,
>    *   Mismatched parentheses,
>    *   Unexpected token.
29c28
<   function lexer($expression) {
---
>   public static function lexer($expression) {
35,36c34
<
<     $precedence = array(
---
>     $precedence = [
41c39
<     );
---
>     ];
46c44,45
<     // for the purpose of comparing only; it's forced to top priority explicitly
---
>     // For the purpose of comparing only.
>     // It is forced to top priority explicitly.
50,52c49,51
<     // tokenizer
<     $tokens = array();
<     for ($i = 0;isset($expression[$i]); $i++) {
---
>     // Tokenizer.
>     $tokens = [];
>     for ($i = 0; isset($expression[$i]); $i++) {
55,56c54,56
<         // noop, whitespace
<       } elseif (strstr($simpletokens, $chr)) {
---
>         // Nothing, whitespace.
>       }
>       elseif (strstr($simpletokens, $chr)) {
58c58,59
<       } elseif (strstr($numbers, $chr)) {
---
>       }
>       elseif (strstr($numbers, $chr)) {
64c65,66
<       } else {
---
>       }
>       else {
70,72c72,74
<     //simple shunting yard algorithm
<     $output_queue = array();
<     $op_stack = array();
---
>     // Shunting-yard algorithm.
>     $output_queue = [];
>     $op_stack = [];
77c79,80
<       } elseif (strstr($operators, $token)) {
---
>       }
>       elseif (strstr($operators, $token)) {
82c85,86
<       } elseif ($token === '(') {
---
>       }
>       elseif ($token === '(') {
84c88,89
<       } elseif ($token === ')') {
---
>       }
>       elseif ($token === ')') {
93c98,99
<       } else {
---
>       }
>       else {
107,109c113
<     if (isset($output_queue)) {
<       return implode(' ', $output_queue);
<     }
---
>     return implode(' ', $output_queue);
113c117
<    * Evaluate an expression given in Postfix (Reverse Polish Notation)
---
>    * Evaluate an expression given in Postfix (Reverse Polish Notation).
116c120
<    *   the expression in postifx notation
---
>    *   The expression in postfix notation.
119c123,124
<    *   The result of the expression
---
>    *   The result of the expression.
>    *
121c126
<    *   When the unknown operator is found
---
>    *   When the unknown operator is found.
124c129
<     $stack = array();
---
>     $stack = [];
127d131
<
129,130d132
<       $tokenNum = "";
<
133c135,136
<       } else {
---
>       }
>       else {
139c142,143
<         } else if ($token[$i] == "/") {
---
>         }
>         elseif ($token[$i] == "/") {
141c145,146
<         } else if ($token[$i] == "-") {
---
>         }
>         elseif ($token[$i] == "-") {
143c148,149
<         } else if ($token[$i] == "+") {
---
>         }
>         elseif ($token[$i] == "+") {
145c151,152
<         } else {
---
>         }
>         else {
152a160
>
Only in math_field/src/Plugin/Field/FieldFormatter: MathFieldFormatter.php
Only in simple_lexer_parser/src/Plugin/Field/FieldFormatter: SimpleLexerParserFormatter.php

In comment #6 you say:

Yes, the original author know about this module.

I want to show how can be extended a Drupal functionality and to add a separate field which value can be simple computed mathematical operations. I was planning to extend (to add) more mathematical operations.

If it is problem to be here you can ask to be removed from either Drupal.org and Git repository.

But in https://github.com/GiorgosK/simple_lexer_parser/issues/1 the original maintainer has said:

I became aware of this module a few days ago but I am not part of any further development (what is done on drupal.org)

That to me is just another sign of you taking actions that are not made with the best intentions and then, the most polite way say this, making mis-statements about your actions. I've given you the benefit of the doubt in this issue and you have not earned it.