Yeah, yeah, I'm coming.
This phrase is one of many odd little loopholes in the English language. It's redundant and meaningless when taken literally, yet it somehow still conveys a message. It describes an event that will take place at a certain point in the course of things, although at what exact time remains uncertain. The implied meaning ends up being I don't know when I'll get there, and possibly I'm in no hurry to get there.
One remarkable thing about language is its resilience. We use it pretty loosely, always inventing new words and finding new ways to use existing ones. There are established rules to keep important things in order, but for most purposes, to save time and effort, we can bend them or just ignore them. This is the only way for language to evolve to match our ever-changing lives.