Hiring an applet JavaScript developer has several advantages. Firstly, they bring in-depth knowledge and understanding of JavaScript, which is essential for creating dynamic and interactive web pages. They can create applets that enhance website functionality, improving user experience and engagement, thereby increasing the website's effectiveness.

Secondly, JavaScript developers are proficient in working with APIs, AJAX, JSON, and other web technologies. This proficiency allows for seamless integration of different services, resulting in a more feature-rich application. They can also create and manage databases, ensuring data security and efficiency.

Thirdly, they can ensure cross-platform compatibility, meaning the applications they develop will work seamlessly across various devices and browsers. This broad compatibility increases the potential audience for your web application.

Fourthly, JavaScript developers can create robust, scalable, and efficient code that can handle high traffic loads and reduce server load. They are also capable of implementing the latest security measures, protecting your web application from potential threats.

Finally, hiring a JavaScript developer can save time and resources. Given their expertise, they can deliver projects faster and with fewer errors, reducing the need for constant debugging and revisions. They can also provide continuous support and maintenance, ensuring the application remains updated and functional.