Motivation for randomness extractors
Here we have $d \ll m$, i.e., we start with a little bit of good randomness, and we end up with a lot.  That's why it's called a "seed": you need something small to get you started, but you end up with a giant beautiful oak tree of randomness.

The other thing to know is that uniformly distributed bits are high-quality randomness, whereas bits with min-entropy $k$ are low-quality randomness.  So an extractor says: if you start with a little bit of high-quality stuff, and a lot of low-quality stuff, you can generate a lot of high-quality stuff.  That sounds potentially useful, at least in theory.

Do cryptographers use them in practice?  Well, they could, but honestly, practitioners usually don't.  Instead, they often use hash functions (and implicitly apply some kind of random oracle assumption).  That's not as principled -- using an extractor would lead to stronger, provable guarantees -- but it's somehow similar.  You could view extractors as providing solid theoretical foundations for this sort of thing.  They also have important implications for a variety of complexity-theoretic results, which I won't try to summarize here, so extractors are certainly interesting to study.