A few questions about overriding the init function

As a beginner of Objective-C, I am very confused about the init function and how and when to override it. So here are a few questions:

• It seems to work fine when the init function is not overridden, so is that just good practice? If so, is it a very bad practice not to do this?
• Suppose I redefine a function because I need to assign a default value to a variable. Should I allocate and initialize all other ivars, including IBOutlets?

Note that I know the syntax:

if ((self = [super init]))
{
    _foo = [[Bar alloc] init];
}
return self;