An underwriter is pretty much the last guy that makes a determination on your loan. Most buyers go with pre-approval from a lending institution, but they are subject to further credit check, debt-to-income ratio verification etc among other checks. Once a contract is signed, the loan application is submitted to an underwriter who approves/rejects loan.
In a very competitive market (2005-2006), it was hard to find a good house for decent price - total seller's market. In that case, to get an upper hand amongst other buyers, lenders would suggest buyer get bank's formal approval - not just a simple pre-approval. I had never heard getting underwriting when making an offer, but perhaps this is also exercised. Hard to fathom, though, that a lender would give you underwriting before even seeing a serious contract in place. But then again, I am not a real estate expert.
If your real estate market is super hot then go for it if a bank provides. I doubt around this time of year, that would be required though. No-contingency offer would trounce any underwriting offer IMHO.