Why does Bitcoin support both compressed and uncompressed keys/addresses? My question is about the difference between compressed keys and addresses, and i know it was addressed in another question but my question is less about the theory and more about how the keys are used practically in Bitcoin.
So heres what I did: I used dumpprivkey for an address 183Tw2TqXKkbk5ZeocTYwmxg8x46ADXb6c in the .7.2 client, and got L5DTi7rgqsPg7Vq1vTU2dP2HWMEsNM5w5JCqEDXXXXXXXXXXXX. When i pasted this into bitaddress.org's wallet details script it returned several things. First, it showed a different address 1FKswUuFsjWKyr4ZPSe3vEpp1iCXXChBZm, but it also showed the address above and said it was the compressed version. I sent like 0.0001 to each of the 2 addresses and my satoshi client only shows a transaction recieved from the 183 (compressed) address.
Can someone please explain in practical terms what the difference is and why they both even exist in Bitcoin? Any help is appreciated.
The reason they exist both, is because Satoshi didn't know about compressed public keys, and it was only recently discovered that they would be possible to use without compatibility problems (support only exists as of version 0.6 of the reference client).

The advantage is clear: their public keys are smaller, resulting in smaller transactions on the network, saving block chain size for everyone.

Update: inside SegWit spends, only compressed keys can be used.