Answer:
Because it helps other people. Just as a carpenter needs tools for making a table, a programmer needs tools for programming. We call these tools "APIs." APIs are just as important in programming as the table-making tools in carpentry. APIS are also the means by which programmers communicate with each other.
Answer2:
Because we want to do something with it. We are interested in some aspect of the program; we want to read it, we want to modify it, we want to run it, we want to test it, we want to debug it, and so on. Some of these things are not just for a particular programmer, but for more people: students or colleagues. That means we need to make it comprehensible to them.
