what is the importance of passing modifiers

Why should I explicitly pass a Modifier in Jetpack Compose when there's a default value, and what are the implications of not doing so?

I'm new to Jetpack Compose and noticed that many composables have a Modifier parameter with a default value. I've tried adding and removing the Modifier argument in my code but didn't see any visual changes. Can someone explain why it's important to pass a Modifier and what happens if I don't include it?

According to your question, You are right it's not necessary to pass a modifier and nothing happens if it is not included.

But, In Jetpack Compose UI is declarative so you can reuse your UI in different ways. Suppose you are using the same design with multiple places with different behaviors. Let's take an example:

@Composable // 1st function
fun Greeting(){
    Card(modifier = Modifier.padding(10.dp).fillMaxWidth()){
        Text(text = "Happy Coding")
    }
} 


@Composable // 2nd function
fun CommonGreeting(modifier: Modifier){
    Card(modifier = modifier){
        Text(text = "Happy Coding")
    }
}

Here are two functions Greeting() and CommonGreeting(modifier).

Greeting() takes the full width of the screen. Whenever you call this function, It will take the full width of the screen. But suppose A scenario where You don't want full-width. you don't need the same padding. You don't need the same behavior. At that time you have to use CommonGreeting() with different behavior with the use of modifier.

CommonGreeting(modifier = Modifier.padding(20.dp)) // padding by 20
CommonGretting(modifier = Modifier.width(130.dp).height(100.dp)) // diff height & width
CommonGreeting(modifier = Modifier.padding(30.dp).fillMaxHeight()) // padding and max height
CommonGreeting(modifier = Modifier) // Nothing - Default

So, that's how you can change the behavior of Commonly used UI. It depends on the designs, layouts, and widgets. If they are used repeatedly with different behavior then you have to go with modifier. It is also a good practice to pass a modifier in the parameter with a default value.

Well, Modifier allows you to decorate the UI. Modifiers help you to do these things:

• Change the composable's size, layout, behavior, and appearance.
• Add information, like accessibility labels
• Process user input
• Add high-level interactions, like making an element clickable, scrollable, draggable, or zoomable.

I hope this helps you well to understand the usage of modifiers. for more - Compose modefiers