Why do we need Top-k (or why not all):

• Think about amazon search, for a given query the user only looks at at most 20 of them because of their low attention and time. So we are only concerned about those top 20 items.
• The value of top_k differs from system to system, for google top_K is around 10 and for facebook it might be 100, assuming the user will scroll a lot

References