Well if you don't pay the quarterly estimated payments you might owe a lot on your tax return and may have a penalty for not paying in if you owe too much.

You must make quarterly estimated tax payments for the current tax year if both of the following apply:

- 1. You expect to owe at least $1,000 in tax for the current tax year, after subtracting your withholding and credits.

- 2. You expect your withholding and credits to be less than the smaller of:

    90% of the tax to be shown on your current year’s tax return, or

  100% of the tax shown on your prior year’s tax return. (Your prior year tax return must cover all 12 months).

Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment.  You pay 15.3% SE tax on 92.35% of your Net Profit greater than $400.  The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare.  So you get social security credit for it when you retire.  

The SE tax is already included in your tax due or reduced your refund.  It is on the 1040 Schedule 2 line 4 which goes to 1040 line 15.  The SE tax is in addition to your regular income tax on the net profit.  You do get to take off the 50% ER portion of the SE tax as an adjustment on 1040 Schedule 1 line 14 which flows to 1040 line 8a.  Turbo Tax automatically calculates the SE Tax and Adjustment.