Why do we use the pH scale to indicate the acidity of a solution rather than simply stating the concentration of hydronium ions?
Because such a statement would be unwieldy.
Which is easier to say: that
Also, historically, we were not always so blessed with computing power. For complex calculations, scientists, and accountants, and engineers, would routinely use log tables. This is not so long ago, maybe 30-40 years. They say that Carl Gauss, probably the most accomplished mathematician in history, had memorized the entire logarithmic table so that he could do complex calculations easily. He probably would have sold his soul for a modern electronic calculator.