When I should split class into struct and struct builder class?

For example, we have next class.

class Request{
public:
  RequestHeader header;
  String body;

  Request(const RequestHeader& header, const String& body); 
  Request(const String& json); 
};

Some articles recommend split Request entity and Request build logic. For example, next.

struct Request{ //RequestDTO?
  RequestHeader header;
  String body;
};

class RequestBuilder{
public:
  static Request build(const RequestHeader& header, const String& body);
  static Request build(const String& json);
};

When we should use this recommendation? At first sight, it is useless complex. Constructor are normal part of a class. Why we should avoid this?