It may be helpful to think of self-employment as being both employer and employee, and keep in mind how payroll taxes work.
In a job-job (W-2, employee) the employer contributes half of the payroll tax: 6.2% of salary/wages up to the current year's Social Security Wage Base, plus 1.45% for Medicare. The employee pays the same amounts, you see that on the pay stub and W-2. For most people, meaning those who don't earn more than the Wage Base, that adds up to 7.65% each, 15.3% total - which is why that's the SE tax rate, because remember SE means you're both employer and employee. [If you're not familiar, the Wage Base is an inflation-indexed amount of salary/wages/SE income that is subject to the 2 X 6.2% Social Security Tax. Google it to see what it is for different years.]
An employee isn't taxed on the employer's portion of the payroll tax, meaning the 7.65% of wages that the employer sends off to the feds on your behalf, out of its own pocket. You don't pay payroll tax on that money, and you don't pay income tax on it either.
To make the SE tax similar - avoiding tax on the "employer" half the SE tax - Schedule SE has you multiply net income from self-employment by 100% - 7.65% = 92.35%. Even though a grade school level math student can tell you that isn't exactly correct as an adjustment. It's close enough I guess - that way you aren't paying SE tax on the SE income that goes straight to the feds as the "employer" half of the SE tax. Then, you take the above the line deduction for half of the SE tax on 1040, so you don't pay income tax on it either - just like being an employee.
As for whether benefits outweigh the taxes, that depends on how much Social Security you and your spouse eventually collect, and how much you rack up in medical expenses covered by Medicare. As others mentioned it certainly doesn't net to a benefit if you just look at the current-year cash in & out.
