what is this: {some code}?

[sitehatchery]

I've deleloped one site using an expression engine template, which is PHP-based. I've seen this on other template programs as well...

They embed a reference to an included file, a database value, or some custom way of coding in curly braces.

What is this? Why do they use it, and why would I want to use it myself versus just using PHP?

[NogDog]

Probably so that they can do search/replace operations on that string, with very little risk of matching the same string in the actual content. For example, {replace_this} is not very likely to appear in your document content, whereas replace_this might (as part of a file name, perhaps?).