Why is it that biochemists often remember the pKa of weak acids, rather than Ka?

Answer 1

It is easier to remember say 3.6 than 2.5 x 10-4

Answer 2

In addition to the good answer by Dr Russ, note also that the pKa is used in equations such as the Henderson-Hasselbalch equation of pH = pKa + log (base/acid). Remember the pKa and we don't need to change from Ka to pKa.