This is basically done to handle the form submission via JavaScript.
For example – for validation purposes
See the below code and see how it can be beneficial:
<script language="JavaScript">
myFunctionName() {
if (document.myForm.myText.value == '')
return false;
// When it returns false - your form will not submit and will not redirect too
else
return true;
// When it returns true - your form will submit and will redirect
// (actually it's a part of submit) id you have mentioned in action
}
</script>
<form name="myForm" onSubmit="return myFunctionName()">
<input type="text" name="myText">
<input type="submit" value="Click Me">
</form>