The relevance of having a debt-to-GDP ratio as a fiscal anchor is generally seen as a good way to measure the capacity of a country to support its debt and to service it. It gives not only the absolute size of the debt, but the size of the debt in comparison to the size of the economy. It's imperfect because measurement of the size of the economy is never perfect. Although measuring the size of the debt can be challenging at times too, this is relatively easier to do.
That's why debt-to-GDP is usually perceived as an easy-to-understand and easy-to-track metric. It's also widely used in international comparisons.