Зачем для application/x-www-form-urlencoded заменять “%20” на “+”?

Для application/x-www-form-urlencoded пробелы должны быть заменены на "+", поэтому вам может понадобиться encodeURIComponent с дополнительной заменой "%20" на "+".
Источник: https://developer.mozilla.org/ru/docs/Web/JavaScript/Reference/Global_Objects/encodeURIComponent

Ответ

Идем на wiki. Для типа данных application/x-www-form-urlencoded указана ссылка на документ HTML 4.01 Specification, Section 17.13.4.1 в котором написано следующее:
application/x-www-form-urlencoded This is the default content type. Forms submitted with this content type must be encoded as follows: Control names and values are escaped. Space characters are replaced by '+', and then reserved characters are escaped as described in [RFC1738], section 2.2: Non-alphanumeric characters are replaced by '%HH', a percent sign and two hexadecimal digits representing the ASCII code of the character. Line breaks are represented as "CR LF" pairs (i.e., '%0D%0A'). The control names/values are listed in the order they appear in the document. The name is separated from the value by '=' and name/value pairs are separated from each other by `&'.