For marketers, custom columns in AdWords can really help for getting more detailed insights into how their campaigns are performing based on non-standard metric targets. Advertisers can now create and track custom metrics at the keyword and ad levels in addition to the campaign and ad group levels, which were available since 2014.
This update is available only in the new AdWords “experience.”
If you try setting up a new custom column or edit an existing one in the old AdWords interface, a notice will come up, letting you know that the new columns aren’t going to transfer over to the new AdWords interface. It’s clear that this is a way to get more people moved over to the new UI.