Six very good reasons for memorising, and not reading words:
It's more fun.
It's good for your mental well-being, a minor achievement, something to be proud of.
It's good for your mental health. (Think brain-training and dementia-delay.)
It's more enjoyable for the listeners: a better performance if you only have to focus on delivery.
It's good manners. In a session you expect other folk to sush and listen while you sing, so the least you can do in return is demonstrate an effort to justify, and show you appreciate, their attention.
And it's not difficult – Those hen party survivors bawling on the late train know all the words of the chart hits because if you like a song and hear it often enough, you pick the words up naturally.