One of the most frequent recommendations I make when auditing Arduino code comes to the difference in use cases for millis() and delay(). This little blurb should help you to differentiate the two and understand why you would use one over the other.

First, millis() is an essential function in programming that returns the elapsed time in milliseconds since the board began running the current program. Unlike other timing functions, millis() is non-blocking, meaning it does not interrupt the flow of your program. Instead, it allows you to check the passage of time at any point in your program. This is particularly useful in scenarios requiring simultaneous tasks or tasks at varying intervals. For instance, if you're operating an LED light while gathering sensor data at different intervals, millis() allows you to do both independently.

Blinking Light Example:

#define LED_PIN LED_BUILTIN
#define BLINK_INTERVAL 1000  // Blink every 1000 ms (1 second)

unsigned long previousMillis = 0;
bool ledState = LOW;

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= BLINK_INTERVAL) {
    previousMillis = currentMillis;  // Remember the time

    ledState = !ledState;            // Toggle the LED state
    digitalWrite(LED_PIN, ledState);
  }
}

While it may require a bit more complexity in the code to store timestamps and check time differences, the benefits of non-blocking multitasking outweigh this additional complexity.

It's sibling, delay(), is used to pause the execution of the current program for a specified number of milliseconds. Unlike millis(), delay() is a blocking function, meaning it stops all other operations on the board for the duration specified. For example, if you are blinking an LED and use delay(1000), the LED will turn on, the program will pause for one second, and then the LED will turn off. While delay() is a simpler and more straightforward function to use, it is best suited to simpler tasks that do not require multitasking.

Blinking Light Example:

#define LED_PIN LED_BUILTIN

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_PIN, HIGH);  // Turn the LED on
  delay(1000);                  // Wait for a second
  digitalWrite(LED_PIN, LOW);   // Turn the LED off
  delay(1000);                  // Wait for a second
}

If the task is time-sensitive or requires simultaneous operations, delay() might not be the best option due to its blocking nature.

I hope this clears up the "why" of each! Please ask any questions