If we are at all interested in a linear transformation, wouldn't we want to know all we could about it? Do you remember, in basic algebra, solving equations a lot? Same thing here. Finding the kernel of a linear transformation, A, is the same as findig all solutions to Ax= 0. Finding the image is the same as finding all b such that Ax= b has a solution.
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Sankaku
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HallsofIvy has made the vital point clear. For the OP, I just want to mention that the concepts of kernel and range are vitally important to all of linear algebra and (later) abstract algebra. They may seem unintuitive at first, but it is worth the effort...
It is well known that a vector space always admits an algebraic (Hamel) basis. This is a theorem that follows from Zorn's lemma based on the Axiom of Choice (AC).
Now consider any specific instance of vector space. Since the AC axiom may or may not be included in the underlying set theory, might there be examples of vector spaces in which an Hamel basis actually doesn't exist ?