Why should I fix E_NOTICE errors?

Question

As a developer, I work with E_NOTICE turned on. Recently though, I was asked why E_NOTICE errors should be fixed. The only reason that I could come up with was that it is be

Answer 1

Because an E_NOTICE indicates an error.
PHP is just too forgiving to call it that.

For example, accessing an undefined variable produces an E_NOTICE.
If this happens often, for example because you're not initializing your variables correctly, and your app is throwing notices all over the place, how are you going to tell the difference between a "variable that works just fine uninitialized" and times when you have really fat-fingered a variable name?

This may trigger a notice but will work as intended, so you ignore the notice:

if ($_GET['foo']) ...

This, on the other hand, will waste half your day while you ignore the notice and are trying to figure out why your "bar() function doesn't work":

$foo = bar();
if ($too) ...

If you don't "fix" the former case, where the variable may legitimately not exist, you can't meaningfully use notices to catch the typo in the second case.

Notices are there to help you debug your app. If you ignore them, you're only making your own life more difficult.