Detailed error logs provide knowledge of the internals of a program. If you're developing a web application open to the public, which is still a common use case for PHP, you don't want an attacker to have this knowledge, as it could highlight a defect in your program that the attacker could use to compromise the confidentiality, integrity, or availability (CIA) of your application.
From the error reporting configuration in the PHP documentation:
This determines whether errors should be printed to the screen as part of the output or if they should be hidden from the user.
This is a feature to support your development and should never be used on production systems (e.g. systems connected to the internet).