Sandburg isn't wasting any time. We get the subject and the metaphor within these first two lines that compare the fog to a cat without using the words "like" or "as." The effect is one that gets us imagining the fog in a catlike way right from the beginning.
Imagine if these lines were a simile instead, and read something like this: "The fog comes as if it has little cat feet." Notice the big difference between the original metaphor and our simile. The former is much more direct and powerful while the latter is a bit subtler and doesn't really fuse the cat and fog into one solid image.
So although we know this is a short poem, we can already see the devices Sandburg is using to most effectively get his ideas across in a limited amount of time.
The arrival of the fog in the first line gives the impression that it's moving slowly towards us. It's not jumping or throwing itself at us, rather it "comes" in a gradual sort of way.
Notice too that the cat-fog seems to be creeping up on us, just like a cat. It has "little cat feet" which kind of sounds like it's tiptoeing towards us without anyone suspecting it.
The enjambment between lines 1 and 2 not only creates a fluid sound for the metaphor, but also gets that image of a cat-fog across without any punctuation interruptions. So the speaker is making the most of his time here while mimicking the gradual movement of the fog. (For more on enjambment, check out "Form and Meter.")
The connotation of the word "little" also serves to emphasize its sneaky, creeping, and quiet arrival. If it instead had "big" feet or just read as "cat feet," the creeping connotation wouldn't be as obvious to us, right?
By the end of line 2 we have the image and metaphor of the creeping catlike fog settled pretty neatly in our minds, and that period serves to provide a moment to pause and fully appreciate the metaphor.
Even if you stop reading here, we're pretty sure the next time you see a fog approaching in the distance you'll think of this metaphor. So Sandburg has done a lot in only two lines.