It's more enjoyable for the listeners: a better performance if you only have to focus on delivery.

Some of the most excruciatingly tedious acts I have ever heard have been from people who have memorized their material (all six songs of it) down the last comma and sing it EXACTLY THE SAME WAY every time. I'd far rather they were trying something different even if it didn't always work.

Memorization is doing something useful if it frees you up to do fresh things with the music - which might be inspiring or might fall flat. In the classical world, Martha Argerich built a career out of that sort of unpredictability, and of course there are folk examples. But there are hell of a lot of NON-examples, and growing. Far too often you get the attitude that because the performer has memorized their stuff, problem solved, they are now an Honest To God Tradition Bearer, and you all better be quiet and listen respectfully, no matter that the singer last had a new insight about that ballad 25 years ago.

Self-righteous pomposity does a lot more to repel newcomers to the folk scene than technical lapses. And you can see just in this thread that terrific memory technique often goes along with bullying arrogance - and particularly with older performers; younger ones are much less likely to think memory skills are a licence for displays of entitled attitude.