Like most other writers, he had to work for a living, so he wrote stories for the pulp magazines to earn money on the side. Clark Ashton Smith's fiction is like Howard crossed with Lovecraft. Many of his stories feature big brawny barbarian swordsmen, like Howard's “Conan.” They also feature evil yet brilliant sorcerers in league with the obscenely horrible Elder Gods. In Smith's stories, the sorcerers always win and the barbarians always lose. Brawn is no match for brain in Smith's conception of reality.
Perhaps because he really wanted to be writing poetry, Smith used a lot of beautiful and poetic imagery. And perhaps for the same reasons, his stories are frequently ironic in tone, as if he never took his job as an affordable pulp writer all that seriously.
In fact, when you realize that Lovecraft's “evil Atlantean priest Klarkash-Ton” is just his pen-pal Clark Ashton inserted into the Lovecraftian universe as a private joke, you have to wonder how seriously any of them took this kind of work!