The Taxpayer Advocate Service helps people through situations that are confusing, or where the IRS is in the wrong. For example, they can help if if you can’t afford to pay your taxes, if the IRS keeps losing your 2010 tax return, or tells you that if you need your refund so badly, you should go get another job. Just not during the shutdown.
The employees who have the power to impose levies on taxpayers, however? They’re still on the job, taking property, bank accounts, and even Social Security checks. You can fight a levy with the help of the Taxpayer Advocate Service when it’s open, but not when it isn’t. Obviously.
Seizing taxpayer property is considered “necessary for the protection of government property” and therefore a function that gets to continue during a shutdown.