Additional thunk for function

What is the purpose for GHC to allocate a thunk for some functions? Why Test.map is not a function, but updatable thunk, which should become equal to the function just after the first call? Here is the details:

% ghc -c test.hs -ddump-stg

==================== STG syntax: ==================== Test.map = \u [] let { map1_sdR = \r [f_sdN ds_sdI] case ds_sdI of wild_sdU { [] -> [] []; : x_sdM xs_sdQ -> let { sat_sdT = \u [] map1_sdR f_sdN xs_sdQ; } in let { sat_sdP = \u [] f_sdN x_sdM; } in : [sat_sdP sat_sdT]; }; } in map1_sdR; SRT(Test.map): []

% cat test.hs module Test where

map f [] = [] map f (x:xs) = f x : Test.map f xs

-- Thanks in advance Victor