Why did developer write "br/" in this example?

Answer 1

Не совсем понятен твой вопрос, лучше бы ты показал пример кода.
Вообще, если речь идет про html, то здесь тег <br> нужен для переноса строки

Answer 2

<br> puts a line break(one free line) before the next content in the document.

There are the alternatives:
<br/> and <br />
They are only there for compatibility with XHTML; to make it work as HTML.

And just so you know, <br> is enough; you don't need the /.