After developing allergies in adulthood, I found avoiding foods I couldn't have while eating out and traveling abroad more difficult. To resolve this issue without needing to give waitstaff my cell phone with a digital note listing my allergies or forcing them to write everything down, I created a small card to give to the kitchen.
On several occasions, I've received "thank you"s from chefs and servers since making them and plan to make more for friends who have similar food restrictions.