Demo days are important. On these days you are given many opportunities. If the customer is new, you have a change to really wow them and get the relationship going on the right foot. If you and the customer have been going through a rough patch a demo is the perfect time to remind them why they hired you and give them new-found confidence in your work.
Demo days are also the worst times for things to go wrong. If someone blows up in your face in front of the customer their opinion of you and your work will plummet fast. And it will be your fault. There is no excuse for something going wrong during the demo. The demo for that day should have been rehearsed in its entirety multiple times. Demos should also have a code freeze attached to them. You don’t want some “bug fix” that morning to screw up what you have practiced demoing the night before.
Demos are the glue that bind you to your customer. If you demo often and well the customer will build their trust in you and keep brining you their business. If you don’t demo often, or worse, have a bad demo they will begin to worry and start to think about how other developers might be a safer option.