The larva feed on dead wood so they help in the recycling of matter in the ecosystem. The adults are a bit more difficult to say as there have been no reported sightings of them feeding.

Actual benefit to the gardener not sure but as they are all part of the circle of life [oops Lion King creeping in again :o)] its a good idea to accommodate them.
The larvae grow to 3-6 cm [1-2 1/4 " in old money] and they look ugly but i'd imagine easily confused with other grubs youd finsd in the garden.

Apparently they love Ginger and mainly seem to live in the SE of England.

http://www.bbc.co.uk/news/science-environment-12132360

I have never seen one in the wild, only on tv, so I don't expect to find one in my garden, much as I'd love to.

Thank you everyone, for your help.

Pretty sure they don't venture as far north as where we live...

I found some living in a rotten tree near RAF White Waltham 1967

http://beneficialbugs.org/bugs/Ground_Beetles/stag_beetles.htm

You also get the lesser stag beetle. Use to be a lot of these near Houghton Mill, Great Ouse near St Ives . Hunts now Cambridgeshire. Use to keep some at St Ivo's School the Biology Class 1961