I’m a European citizen. I bet it differs from country to country, but where I live, there are very rare cases they have you sign them. I believe it’s when you don’t actually pay on the spot, but rather give them a permission to withdraw that amount of money from my account later (SEPA Lastschriftmandat). Not sure why you would opt for that option as a business. To a customer it works the same, except the latter option takes longer.