Your stated wage is calculated on scheduled hours. But you're not paid for all the time your job actually takes.
If you're salaried at $75,000 and work 40 scheduled hours, your stated rate is $36/hr. But if you actually work 50 hours, commute 1 hour/day, and spend 30 minutes decompressing, that's 57.5 real hours per week — dropping your rate to $25/hr before expenses.
Commuting costs, work clothing, lunches you wouldn't buy at home, childcare needed because of work — these are all job costs that reduce your real take-home. A $2,400/year commute on a $75,000 salary is 3.2% of gross, or about $1.15/hr reduction in your real rate.
Your stated hourly rate is usually gross, not net. Federal + state taxes typically reduce take-home by 20-30%. The calculator accounts for your effective tax rate, not your bracket rate, to give a realistic after-tax number.
Your true hourly rate is most useful for comparing job offers (a higher salary in a farther city might pay less per real hour), deciding if a purchase is worth X hours of your life, or evaluating whether remote work's eliminated commute changes the math enough to matter.