You can get an Arduino Nano clone for considerably less than $5 on eBay. The cheapest price I can find a local Pi Zero for on eBay is $24 AUD (without an SD card).

I use Arduinos and similar boards to drive addressable LED strips. A lot of these strips require extremely precise timing, which is difficult or impossible to get from a Pi for some strips.

From another perspective, the limitations of the Arduino boards (e.g. RAM measured in kilobytes) force you to think harder about how you want to solve a problem, which can be quite an enjoyable challenge.

To get started with an Arduino you install the IDE and plug it into a USB cable, load Blink and hoorah your first program is already running. By comparison a PI is far more mucking around https://projects.raspberrypi.org/en/projects/raspberry-pi-se...

Because Rpis and Arduinos are completely different. Arduinos have a microcontroller onboard, not a microprocessor. Along with ADC and PWM to simulate DAC. You need a real-time system like Arduino in order to properly interface with sensors and actuators.