Use of retain in initWithCoder?

I am reading up about Encoding and Decoding and I noticed that sometimes people miss the retain off the end, I have also noticed that retain is sometimes used on some varables but not others. Can I ask ...

(1) What is the purpose of this retain and why is it sometimes not needed?

(2) Does the use of the retain imply that I need to match it with a release, and if so where?

- (id) initWithCoder: (NSCoder *) decoder {
    name  = [[decoder decodeObjectForKey: @"CardName"] retain];
    email = [[decoder decodeObjectForKey: @"CardEmail"] retain];
}

or

- (id) initWithCoder: (NSCoder *) decoder {
    name  = [decoder decodeObjectForKey: @"CardName"];
    email = [decoder decodeObjectForKey: @"CardEmail"];
}

gary