Javascript-how do we understand square brackets described in the syntax in the manual?

When I look up some documents, I often see functionfuncname ([arg1 [, arg2 [..., argn]) {statements} or the rhinoceros varname_1 [value_1] [,..., name_n [value_n]. Similarly, it is often found in the php or css manual... I often see this method when I look up some documents.

In the rhino book
function funcname([arg1 [, arg2 [..., argn]]]){ statements }

Or a rhino book
var name_1 [ = value_1][,..., name_n [= value_n]]

For example, I often see this square brackets in the php or css manuals. How can I understand this square brackets?
Why can't I write it as follows?

function funcname(arg1, arg2, argn){ statements}

var name_1 = value_1, ..., name_n = value_n

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I often see this method when I look up some documents.

In the rhino book
function funcname([arg1 [, arg2 [..., argn]]]){ statements }

Or a rhino book
var name_1 [ = value_1][,..., name_n [= value_n]]

For example, I often see this square brackets in the php or css manuals. How can I understand this square brackets?
Why can't I write it as follows?

function funcname(arg1, arg2, argn){ statements}

var name_1 = value_1, ..., name_n = value_n

Square brackets refer to optional parameters, that is, they are optional parameters, and there is no limit on the number. You can enter 0 or N parameters.

Optional parameter. Optional.