The examples given are instances of present perfect tense contrasted with past tense. The present perfect generally emphasizes the present importance of past events, as opposed to a simple past tense, which in English may denote a completed action. Without context it’s hard to read the definition of friends here, but for the sake of argument, the simple past tense (“I knew him”) has to mean that the acquaintanceship itself ended at some point.

The third example (“He has played really well”) seems to summarize the past (leading to a present state of affairs), while a simple past tense (“He played really well”) might point to a specific (completed) past event.