ELI5: What is the point of buffer overflow attacks?

Every video/material I read talks about HOW to do a buffer overflow.

I dont understand WHY to do a buffer overflow.

Say I have a malicious code "DROP TABLE XYZ"

1. Why wont the program/OS run the code if I input it to the buffer as is.

2. But if I input the code in the buffer and overflow it and then make the return address to point to the line in the buffer that has the malicious code "DROP …

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