Q: I just started my practice and I’m not making any money yet, but my CPA wants me to start paying estimated taxes. Why?
A: This is one of those tax things that seem to make absolutely no sense. Your CPA is being extra cautious, but he/she is right.
Here is what the IRS says:

Sole proprietors, partners, and S corporation shareholders - You generally have to make estimated tax payments if you expect to owe tax of $1,000 or more when you file your return.

Corporations - You generally have to make estimated tax payments for your corporation if you expect it to owe tax of $500 or more when you file its return.

In other words, if you will be making a profit by the end of the year, it’s best to start now by paying estimated taxes. I know cash is tight as you start out, but you really don’t want to have a big tax bill at the end of the year. Pay quarterly whatever you can, to lessen the impat at the end of the year.