You should level incoming candidates based on their performance during interviews, their prior projects and portfolios, not what level they were labelled as at a previous company. I don't care if you were a Level 38 at Foo Inc., I'm going to interview you and compare what I think you're capable of to the levels at my company and make a recommendation based on that.

Most interview processes I've been a part of had a target level for the candidate, with the possibility of leveling them up or down based on the outcome of that interview.

There are other reasons why leveling can be important - the expectations of the candidate around title and comp, for instance.

This is why sites like levels.fyi exist. They are surely not perfect and I don't know much about how the data is collected or verified, but it's a question that people want answered.

Interviews should be pass or not pass. Leveling is a bet and a risky one based on a 45min interview. It is probably best to ask the candidate what they want and tell them where the options are to help that bet pay off.