What are PHP versions and how do they work?

i always thinking about this for example i using php version 7.1 but its same like other versions. Why PHP developers always building new versions ? What change ?

In fact with upgrades, they improve security and They replace the best ways :\

PHP 7.3 is now out they add in new functions and remove deprecated calls etc so would be best to stay current and read the change logs on the different versions to see why