Your marginal tax rate answers one question: of the next pound you earn, how much do you keep? It is the rate on the top slice of your income, not on all of it. In a banded system such as UK income tax, crossing a threshold changes the rate only on the income above it, so a pay rise never leaves you worse off through income tax alone.
The average or effective rate is a different figure: everything you pay divided by everything you earn. It is always lower than the top band's rate in a simple banded system, because the lower slices are taxed less or not at all. Use the effective rate to budget and the marginal rate to judge a raise, overtime or a pension contribution.
The marginal rate is not always the headline band. In the UK the personal allowance shrinks by £1 for every £2 of adjusted net income above £100,000 and is gone at £125,140, according to GOV.UK. Across that stretch each extra pound is taxed at 40% and also pulls 50p of allowance into tax at 40%, so income tax takes 60p of it. Employee National Insurance at 2% above the upper earnings limit lifts the total to 62p.
So the useful figure is the combined one. The take-home pay calculator shows your marginal rate beside the effective rate, counting income tax and every contribution charged on the same pound. It measures it by working out the deductions on one more unit of pay, rather than reading a band off a table, so tapers and credits that phase out are caught as well.