If you interpret the start and end tags of the child elements as syntax indicating the type of each value, then those tags are analogous to, say, the quotes that enclose a string literal. In other words, in

    <foo>hello</foo>
    <foo>world</foo>

the <foo> and </foo> serve the same purpose as the double quotes in

    "hello",
    "world"

with the added benefit that the type system can be much richer (i.e. not everything is just a nondescript string value).

And you don’t even need a comma to separate the values! ;)