The answer to this will help in moderating our habits, and behavior.
There is no point in reading about karma when we don’t use it for our betterment.
a simple derivation is “good karma keeps you happy” and “Bad karma leads to unhappiness by accumulating guilt in the mind”
Unexpected things happen without our involvement or knowledge. People can escape from unnecessary guilt by transferring the blame to past karma. (this concept is difficult to digest for a lot of people who do not believe in karma)
We often suffer from guilt because of unexpected events happening in life and we struggle to get a rational explanation for them, exactly for these situations rationalizing with karma helps.
Though karma cannot answer all life situations, it is a conceptual tool to help us reduce the burden of guilt.