Not really related to the topic, but I was just wondering why you have to do this ? In the UK, a driving licence is valid everywhere !
Still a bit off topic...but not so much that I don't think this is applicable.
In some states - like California - your residency means a whole lot more for you than just living there.
I had to become a California resident as part of my tuition waiver for grad school. I will have to prove my residency - i.e., my intent to live permanently in the state of California - for my tuition waiver to kick in.
Also, if you want to vote in current elections that your city is having (for example, all of the various very controversial elections California has recently held!) it's vital that you are a resident of that state. If you don't change your residency, you can still vote absentee in your old state - but if you want to make a difference in where you are living NOW and what budgets will impact you now (cf., again, the recent ballot measures that did not pass), it's important to become a resident as well.
I had this huge list of things I must make sure I did within 10 days of arriving in California for my residency to be considered. Get a driver's license, establish bills in my name, change my car registration over (which...CA has the strictest smog laws in the country, so you can't just bring any car and expect that you'll be able to register it there), get a bank account (not good enough just to switch your address - I had to actually open a NEW bank account in California). This summer I'll have to show all my move documents, I'll have to prove memberships at various clubs, grocery stores, the library, etc. Its intense.
Moving there for a job would be a little different, but it's still an extremely expensive (and sometimes difficult) place to live. You absolutely cannot survive without a car there.
I'm not sure where I'll go next when school is done. Moving across the country is expensive and time-consuming!