I think this is mostly a US thing. Why use yearly salary? You’re not paid once a year, are you? Most likely once a month. Referencing monthly salary makes much more sense.
“I’m making 50k”. Great, now I have to guess - dollars? Monthly? Yearly? If yearly then what’s the monthly paycheck? Net? Gross?
Because your yearly income dictates your tax bracket.
This isn’t how tax brackets work. Say we have 5%, 10%, and 15% in a system similar to the US with completely made up numbers. Say you make $99k, your first $33k will be taxed at 5% ($1650), your second $33k at 10%($3300), and your last $33k taxed at 15%($4950) totalling $9900. 9900/99000= a 10% effective tax rate. The real system is more complicated, but this is what it boils down to.