It's not just allergies, but intolerances. I have an intolerance---i.e., an issue when consuming them that has nothing to do with an immune system response---and it's the #1 condition affecting my life.

I cannot eat out of virtually any commercial kitchens (restaurants, food trucks, etc.) because cross-contamination is everywhere. The great majority of foods at the store that say "spices" have paprika in them. I know this because I managed to get a sneak peek at an ingredients list for a major spice company that makes the seasoning mixes for thousands of other food manufacturers. And potato starch shows up EVERYWHERE these days.

Nightshades, particularly tomatoes and potatoes, are cheap and so are used as fillers in a TON of foods. Avoiding them is next to impossible when eating outside the house. So I can rarely socialize with friends at restaurants unless I eat beforehand/after and then during the meal sit there like a chump sort of picking at a bread roll or whatever. It sucks.

Eat them, even trace amounts, and my life basically goes to shit for several days, often culminating in seizures. Not fun.

But besides people like me (there are dozens of us!), a lot of people who avoid nightshades do it for anti-inflammatory purposes, or to tame their arthritis.