When are inner covers worthwhile?

[omnimirage]

I've never been taught to use an inner cover, rather to use a "hivemat" instead. I see a lot of people use inner covers, but surprisingly, I can only find a limited information on their actual uses, advantages and disadvantages. I'm wondering if it'd be worthwhile to invest in inner covers for my hives.

Under what circumstances would it be best to use an inner cover? Under what circumstances would it not be best to use one?

[Michael Bush]

Inner cover:
http://www.bushfarms.com/beesfaqs.htm#innercover

[PhilK]

Hey omnimirage, this seems to be a US thing. Doesn't seem like a lot of beeks over here use them... none of the ones I know anyway! Hive mats have been recommended to me too.

[omnimirage]

Good to know.