Many sensors have convenient linear outputs.
Others, such as thermistors and audiotaper potentiometers, produce logarithmic outputs. While it's possible to straighten out a log curve in software, it's often easier to deal with it in hardware. If you play this curve against a log input, you can effectively straighten the curve. The 4046 isn't the only game in town, either. For sensors that convert some physical property such as humidity or proximity into a variable capacitance or inductance, pulsin is a natural candidate for sampling their output via an oscillator or timer.