I have magical insect repellent that prevents nuisance1,2 insects from landing on me, biting/stinging me, or congregating within, say, 1m of my face.
Would I still want to employ insect screening on areas that are always air-open to the outside? If so, why? For example, why would I want to screen in a porch, deck or swimming pool? (Screens between indoor and outdoor spaces still have the obvious use of reducing ingress into my living area, so I'm not asking about those. If it makes a difference, assume my insects are similar to those in the mountains of California.)
Let's assume I am not also trying to keep certain insects in. (That is, I am not, for example, building an open-air butterfly conservatory.)
(1 Particularly flies, mosquitos, gnats, ticks, etc.)
(2 "Non-nuisance" insects are unaffected. Butterflies, certainly, probably dragonflies, possibly moths and/or tarantulas, maybe others. It's magic, so it might even be able to change behavior based on the individual or even their mood.)
Edit 1: The intent was not to get too hung up on how the magic works, just to assume that it does, but since some folks are answering based on assumptions as to how it works, perhaps I should clarify... The magic is wearable (i.e. affects a person/animal, not an area), extremely persistent (lasts for years), it's inexpensive enough that most middle-class people wouldn't think twice about using it on pets, and one can assume that guests will "bring their own". For most purposes, and particularly from a practicality standpoint, you could substitute a really effective, inexpensive science-based rub-on/spray-on repellent.
Edit 2: I intentionally left out details in the original question, because I like seeing generally applicable answers (that's the purpose of this site, after all!). So, please don't take this as invalidating any existing answers; after all, my specific answer might be "because screening in porches is just what people do".
That said... this specific porch is a small part of a much larger deck; far too large to screen in the whole thing (most of which is anyway open to the sky). The main eating areas are in the open-to-sky areas; eating in the screened-in part would be intimate and inconvenient (it's on the opposite side of the house from the kitchen). The screened section is the only part with a normal-height roof, and is also facing a mountain slope at fairly short distance, so "shade" isn't an issue.
Any answers that address this specific situation would be awesome, however, looking at it more closely, I realize there are a number of aspects of this porch that aren't exactly practical, so for my specific case I'm leaning rather heavily toward "because it's expected" as the reason. Thus, unless I happen to get a particularly brilliant answer for my specific case, I will accept one for why this world would generally bother to screen things, so keep those coming also!
Lots of good answers, often with recurring themes; thanks! In the end I awarded Ash (largely for having the longest list of reasons), although Mike Serfas definitely gets an honorable mention, and I want to give a shout-out to Radovan Garabík for the mention of keeping bugs (and birds, squirrels, etc.) out of your swimming pool.
Ultimately, for my specific instance, I'm going with a combination of "tradition" and keeping crud out. There's a huge area of deck, but more than half is open to the sky and can take advantage of rain to do some "natural washing". The part that's screened is fairly enclosed even without screening, and is much more "intimate" than the other areas; keeping insects entirely out because someone might want to take a nap there is plausible, though perhaps an accidental benefit.
I got myself into this mess, after all, by giving my setting a screened-in porch without stopping to ask why I was screening in that, and only that, specific area. Given plenty of reason why my story-world would in general still screen in outdoor areas, I see no problem making that the in-universe explanation as well 😄.