Analog signals have been part of industrial control systems for a very long time. The two most common signals are 0-10V (“voltage”) and 4-20mA (“current”), although there are a wide variety of other voltage and current protocols. These signals are called “analog” because they vary continuously and have theoretically infinite resolution (although practical resolution is limited by the level of residual electrical noise in the circuit).
Measurement sensors typically provide analog output signals, because these electronic circuits are well-understood and the designs are relatively economical to produce. But that doesn’t mean it’s easy to design and build a good-quality analog sensor: in fact it is very difficult to engineer an analog signal that is highly linear over its measuring range, has low noise (for high-resolution), is thermally stable, (doesn’t drift as temperature changes), and is repeatable from sample to sample. It takes a lot of careful engineering, testing, and tweaking to deliver a good analog sensor to the market.