A Joomla! migration gives everyone a new opportunity to:

• Revisit site goals and rethink its purpose
• Clean up "great ideas" that never saw the light of day
• Inventory third-party extensions that may have been installed but not used
• Simplify and clean up old articles, menus and other content
• Create new functionality on the site
• Change design and navigation
• Add accessibility to disabled visitors
• Make the site mobile-friendly
• Improve search engine optimization
• Utilize new features you may have been using third-party extensions for
• Insure your data against security breaches by having all the latest software

Features and usability enhancements
Typically new life cycles of Joomla include new features and usability enhancements. These sometimes can eliminate the need of a third-party extension and enable the increased use of Joomla core components. Read more about features Joomla 3 features.

Technology Changes
Significant changes have taken place! Major releases of Joomla have restructured the data and require a “migration” as opposed to an update or upgrade. Technology changes have also been made within the versions of PHP and MySQL running on your hosts server. These changes force all content management system (CMS) technology to change with it.

Security
When a CMS changes its structure, a migration is required. The old version is no longer supported. This creates potential problems with security as security patches are discontinued and older versions of Joomla are not maintained.

Many hosts don’t want older software scripts (Joomla is a script - all third-party extensions are scripts) running on their servers because security is compromised as software ages. They have to update their servers to remain secure. More and more older versions of Joomla cease functioning because of server upgrades on the part of the host.

Source : https://docs.joomla.org/Why_Migrate