I am developing web application using MERN stack but instead of using mongo db I am using mysql as a backend database. But do i need to install mysql workbench if I already have xampp installed. because i need to start xampp server for mysql. I am using sequelize library. I am new to the development.
tl;dr: No.
XAMPP is a bundle which gives you:
MariaDB is a database server (compatible with MySQL) which stores and processes data.
PHP and Perl are programming languages which let you write software which interacts with the database. You need a programming language to do web development which is database driven. Other programming languages are available. You tagged this question sequelize.js which is a library for Node.js which you can use instead of PHP or Perl.
phpMyAdmin is a database client. It gives you a GUI view of your database. It lets you manipulate the database, examine the content and structure of it, and run queries (including testing queries that you plan to use in the software you are writing with PHP/Perl/etc). This is useful but not necessary.
MySQL Workbench is also a database client. It does the same job as phpMyAdmin. The feature set is probably a bit different. Like phpMyAdmin, it is useful but not necessary. You might prefer it to phpMyAdmin. You might prefer a different database client entirely (personally I'm rather fond of Beekeeper Studio).