When Garth Ennis said he wrote Troubled Souls primarily to get published, what exactly was it that helped him?

Garth Ennis is probably my favourite comic writer. I haven’t been able to read Troubled Souls but I saw online that he wrote more to get started in the business of writing. I’m interested in writing as well and I was wondering if those who have read it could help explain what the work had that made Garth Ennis a long lasting writer.