Essential, air travel has morphed such that any window, or an aisle seat, are considered premium - at extra cost.
So, you can book your seats months in advance and pick seats next to each other but as the flight date approaches, the quant models for the seating are overlaid and seats (never guaranteed) are rearranged. You had a window, middle and aisle for Husband, Wife, Child in your original intinerary? Congrats, each of the three of you are in middle seats in varied rows anywhere in the plane.
Flyers who have status or who are willing to pay an upcharge to guarantee an aisle seat or a window seat have shuffled the quant deck. If you aren't in either of those categories, your seat is now on open play and up for bid.
My ILs travel maybe once every five years by plane. For their latest trip, when I purchased their tickets, I got guaranteed seats for them (including purchasing a middle seat so that they could spread out, my FIL having some mbility issues) and instructed my MIL to not give them up or trade them. Sure enough, a family came along and played the "but our children have to be with us! So you move to give us your premium seats and take our cheapest possibly seats back near the lavs! Because we are a FAAAAMMMMMMILLLLY! and therefore deserve more than you selfish old people without kids!"
My MIL who has the sweetest voice, simply looked at them and said "noooooo". When the father of the FFAAAAAMMMMMILLLLY tried again, my FIL - a man of very little English but remarkable turn of phrase, said "I know a ten cent millionaire when I see him."
The FFFAAAAMMMMMILLLLY whined to the flight attendant about the "spare" middle seat and how much better it would be for them if they could have those three seats for their FFAAAAAAMMMMMILLLLY. The FA, taking her dignity in her hands, approached my FIL. Who responded: "Ask my wife." MIL came back her her sweet sweet "no......"
The FFFFAAAAMMMMMILLLY groused for the rest of the flight, but I'm sure that helped them start their trip out wonderfully. I really only care about my ILs. They've worked their whole lives and raised their own FFFAAAAMMMMMIIIILLLLLLYYY so I feel no compunction about making sure that they get as much of the good life as I can help with.
Next time, in spite of their protestations, I'm upgrading them to first.