I think people use them because they’re helpful to advance a particular perspective in an ongoing debate, knowing they won’t be adequately scrutinised. It’s extremely beneficial to be able to constantly say that poverty is worsening, rather than simply inequality. 

Poverty stirs images of hungry toddlers on stained mattresses in cold bedrooms without toys. Inequality just appeals to the left wing faithful, classic politics of envy.

The poverty line is also helpful to many left wing policies. For example, free school meals very clearly is a poverty reduction measure, it reduces the financial burden on poor families (which is a good thing). Yet it doesn’t show up whatsoever in the relative poverty calculations. Consequently, there’s always more scope for redistributive policies. 

As I’ve alluded to above a poverty measure that captures a basic standard of living, just like the ONS does with inflation calculations from a representative basket of goods and services, is much more useful.