It simplifies the thinking, planning and shopping and that's what you're paying for.
If you like thinking about food and browsing recipe books/websites, you probably won't see the appeal.
If you have a good variety of shops nearby, and can buy all sorts of spices and seasonings cheaply it won't be as attractive as if you live in a rural market town and your local supermarket is Tesco or Waitrose and all the herbs and spices are Schwartz or Cooks Ingredients at five times the price.
I've never been tempted because we normally have 80% of the ingredients required in anyway, so it would be a waste of money.
I did try Simply Cook, but a lot of the seasonings sent were full of sugar, which I didn't like but I wouldn't pay full price anyway, because it is ridiculously expensive - you're basically paying almost the same for a tablespoon of a spice paste that you would a whole jar in the supermarket.
Another option for anyone who likes the idea but not the cost, amount of packaging or not being able to choose your own ingredients, would be to look at the recipe section of a supermarket website (I think nearly all of them that do online shopping has them) choose what you want and press the 'add to online order' button. You'll be able to remove anything you already have, adjust quantities/brands/pack sizes etc and complete your shop with toilet roll, cereal, etc etc. So you get a similar experience, with less packaging and likely lower cost.
Because whatever they say, it's not a cheap option, especially at full price. £4 per portion for a home cooked meal is probably two or three times what comparable meals cost, unless you're in the habit of buying bulk packs of vegetables instead of the loose amount of what you actually need and premium level spices, sauces etc and only using them once.
If you're the type of person who cooks regularly and keeps a half decent store cupboard, you'll be buying the cheaper larger packs, using them all and the cost per meal will be almost nothing. I've just had to replace all my spices because they inexplicably ran out all the same time and it probably cost around a fiver for
coriander
cumin
chilli
garam masala
paprika
turmeric
and a couple of others that I can't remember.