Atma's Impaler is one of the most cost-effective items in game, surpassing Pickaxe on any level 18 AD champion (without HP items).
Along with being built from a gold-gen item (Avarice Blade), the cost-effectiveness is increased.
Atma's Impaler can be combined with items such as Frozen Mallet and Warmog's Armor to utilize both damage and survivability. These builds are known in the community as "Fratma's" and "Atmog's", respectively, "Fratmog's" for both at the same time.
Atma's Impaler is one of only a handful of items beneficial to an attack-based character that provides armor, the only others being The Lightbringer, which along with not being available on SR, provides less: armor, late-game AD and no crit chance; and Iceborn Gauntlet which (while not giving any direct stat boost to AD) provides bonus damage on-hit damage on basic attacks and provides more armor at a higher cost.
However, some champions can use this armor in synergy with AD magic resist items for a balanced defense, and making use of the Atma's Impaler's critical chance to boost items like Infinity Edge.