TL:DR; Classes should have just one responsibility and one reason to change.

Problems

Coupling
Code Duplication
Low Cohesion
Single Responsibility Principle violation

Solutions

Extract class

Context

We create classes to fulfill responsibilities.

If an object does too much, it might change in different directions.

Sample Code

Wrong

class Webpage {

  renderHTML(): {
    renderDocType();
    renderTitle();
    renderRssHeader();
    renderRssTitle();
    renderRssDescription();
   // ...
  }
  //HTML render can change

  renderRssDescription() {
   // ...
  }

  renderRssTitle() {
   // ...
  }

  renderRssPubDate() {
   // ...
  }
  //RSS Format might change

}

Right

class Webpage {

  renderHTML() {
    this.renderDocType();
    this.renderTitle();
    (new RSSFeed()).render();
    this.renderRssTitle();
    this.renderRssDescription();
   // ...
  }
  //HTML render can change
}

class RSSFeed {
  render() {
    this.renderDescription();
    this.renderTitle();
    this.renderPubDate();
    //...
  }  
  //RSS Format might change
  //Might have unitary tests 
  //etc
}