Why was Bing Bong collecting memories?

It is never explained in the movie. However, his appearance and his actions seem to me like they were intended by the movie's creators to make him seem like a villain:

Let’s go back to Bing Bong’s introduction: he’s first spotted by Joy (Amy Poehler) and Sadness (Phyllis Smith) in long-term memory. This is the area of Riley’s mind that is supposed to be off-limits, populated only by blobby mind workers that perform maintenance and provide valuable upkeep and categorization. But this is where Bing Bong has found himself—he’s stealing Riley’s memories, hoarding his favorites in a “make believe” bag that also includes (in no particular order) a shoe, a kitchen sink, and a live cat. He’s dressed shabbily, like a hobo in an old Hollywood movie: torn, fingerless gloves and patches on the elbow of his jackets. Given his working knowledge of the Train of Thought, you can tell he’s been riding the rails. All he’s missing is a polka-dotted bindle. (We saw the first hour of Inside Out at a press event earlier this year and were convinced, for many weeks, that Bing Bong was going to turn out to be the film’s villain; the filmmakers admitted to us that this was purposeful and a way to keep the audience on their toes.)