Typically, boiler plate implemented by a standalone function is not easily discoverable, eg. no clear convention where to put it, does not look idiomatic on some usages syntax habits, and not suggested by auto completion in top results.

Extensions are searchable by syntax and naturally in a namespace, largely diminishing the risk of put it in in a single use function that will be quickly forgotten, and might seem obscure.

The code itself might not be much shorter, but you don't introduce code looking weird, if it's re-usable it's now discoverable, and if your boilerplate contained a lot of micro adjustments on a larger glue code, this glue code is now shorter and way clearer.