Normal people in normal jobs --- hell, ace (non-founder) devs at startups taking W2 wages --- have most of their income subject to payroll taxes (most notably FICA). These taxes are a significant chunk of all your withholdings.
However, if you and your partners own an S-Corp, you can choose to pay yourself not only in direct wages but also as cash distributions (think "profit sharing"). Unlike wages, distros are not subject to payroll taxes. You can skip out on your FICA payments for all the income you elect to pay as a distro.
In an LLC, all of a company principal's income is paid as distros (you can't formally be an LLC principal and take W2 wages). But that income is all subject to self-employment tax. It's only in the weird little blip between LLC and C-Corp that this tax avoidance trick exists.
There is no legitimate reason why an LLC principal should have to fully fund Medicare and Social Security, and a C-Corp director should have to fully fund Medicare and Social Security, but an S-Corp shareholder should be able to carve out some (or even most) of their income to avoid those taxes.
To my eyes, this is a loophole in the purest sense of the word, and I'd be happy to see them kill it.