Hi rahul,

because they developed that code/ modules for some specific feature purpose and might be that code depends on some other base modules and day by day odoo provides update for existing code so all feature already available and odoo already provides basic and standard facility to existing code so if we have to add some extra feature then we do not change existing code because it depends on some module and it conflict existing feature so for all these reason we have to customise odoo.

i hope this is just little bit information so you have to properly study odoo documentation for reference.

https://www.odoo.com/documentation/12.0/index.html

thank you