I have anaphylaxis-level allergies to milk, peanuts, tree nuts, and seafood (all fish, all shellfish).
One thing I have learned is that people who do NOT have severe food allergies, or don't have to deal with them due to, for example, a family member, often have no appreciation of how challenging they actually are in practice. Some allergens, like milk, can be pervasive once you actually start reading and asking about ingredients - they are in things you never would imagine they would be in, and often can't figure out why it IS in it. Cross-contamination issues at restaurants are significant. Just ordering something that appears to be safe off the menu, but not notifying the staff of the allergy, is usually a little like playing Russian roulette.
And not eating out is not always an option. For example, a few weeks ago I had to travel for work. Not traveling was not an option. Eating in restaurants was the only option for food. I did my research ahead of time and identified four chain restaurants that are are "safe" for me to eat at at home (as in, they can accommodate willingly), figured out where they were and identified how I would get there. They were where I ate during the trip, they accommodated just as they do at home and I had no issues.
A couple examples of why just ordering off the menu is not always an option:
At Tony's in MK, I had checked the menu in advance and knew I wanted the spaghetti with meatballs. Seems safe, right ? Except that their meatballs have a dairy product in them. Because I had an allergy notation on our ADR and mentioned it when we checked in and to our server, a chef came out to speak with us, per Disney policy. The chef explained the milk-in-the-meatballs situation, but then explained that their turkey meatballs do not have milk and were safe for me, would they be ok ? Sure ! So I had spaghetti and turkey meatballs. If I had just ordered off the menu what looked like it was safe I would have ended up in anaphylaxis and a trip to the ER.
During my recent work trip, at one of the restaurants I ordered steak. Normally it is grilled with butter. I asked if they could prepare it using something like olive oil instead, which wasn't a problem. The chef then passed a question via the server: did I want it pan fried or cooked in the oven, as they could not make the grill safe since seafood items get grilled on it too. I chose pan-fried. Again, if I had just asked for the steak prepared with olive oil but not explained the allergies, there would have been a cross-contamination issue.
One of the ways I make it easier when dining out at carefuly chosen restaurants is I bring a 4x6 index card called a "chef's card" that lists my allergies and a message about avoiding cross-contamination. I give the card to the server or chef when explaining my allergies, and the card can be taken back to the kitchen. I have gotten a lot of positive feedback about the card, including more than one chef who has come out to talk to me and thank me for having brought it. I highly recommend such cards for those who have severe food allergies.
Sometimes my food takes longer. Sometimes they make a mistake, realize it, and have to redo my food entirely, so it really takes longer. That is ok. i would rather have safe food than fast food. Besides, I have dining companions or a smart phone to occupy my time. Sometimes I can't have exactly what I want because of a hidden ingredient, but usually there is a substitution that will work. I eat a lot of plain food when I eat out, as lots of sauces and such are not safe. I am ok with that as well.
I have ran into situations where a restaurant could not accommodate me. The most frustrating was when my connecting flight at Chicago O'Hare airport was canceled and I was put on a later flight -- about 8 hours later. I was given vouchers for lunch and dinner. great. Except that every single one of the sit down restaurants I tried to eat at REFUSED to serve me because they could not accommodate my allergies. Fortunately, there IS a McDonalds in the terminal I was in, and I CAN eat some things there, so that is where I ate. The McDonalds manager was awesome -- made sure the staff preparing my food changed gloves and had a clean work surface, and also tried to think of things I could get that would use up the voucher value (I came nowhere close for the dinner voucher

) Since that incident I travel with even more food in my carry-on as not all airports have McDonalds.

I also research the dining options at all airports I will be in for a trip in case flights are delayed. Pretty much means I bring enough food for two safe meals

FWIW -- many airport restaurants have extremely limited kitchens, often just reheating food prepared offsite, so may have limited or no ability to make modifications.
SW