You can either do a mileage deduction or you can proportionally deduct a share of gas, maintenance, and repairs. (And also depreciation I think, although I haven't done that in a while.) I've always found the mileage method to be better, but your needs may vary. (What you can't do is switch back and forth year-to-year.)
Any deduction takes away from your taxable income. Meaning the benefit of the deduction is not paying taxes on that amount. The rate depends on what bracket you're in, but we also have to pay 15.3% Self-Employment Tax. (Medicare and Social Security.)
If you're in a 12% bracket (earning under $41,775 as an individual), we're talking about 27%. Times 62.5 cents per mile, every mile you claim is almost 17 cents off your tax bill. So that 100 mile round trip, if documented, is $17 more cash in your pocket.
(If you or your spouse are making considerably more then your tax rate is higher, meaning the deduction is even better.)