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?
It’s not as useful for day-to-day budgeting as a more granular one, but people generally only look at their finances closely once a year at tax time and so it’s a good point of comparison for that; get a sense of how your financial life is evolving.
It’s also the number you’re asked for on tax forms, other financial forms (loans, financial aid, bank accounts), questionnaires (though you can lie or ‘prefer not to say’ on those)… comes up a lot, basically.