I tutor kids in maths (mostly from grades 7 to 9), and most of them struggle greatly with understanding why the fuck they suddenly need to use letters. They tell me that this way they aren’t doing anything concrete, so it feels even more useless and confusing than arithmetics did. I always give them the same explanation: I tell them that mathematicians are just very, very lazy people. Their whole goal is to make every calculation needed for practical reasons take shorter, for every proof to be written as consisely as possible. And doesn’t that sound half bad, does it? So because mathematicians are soooo lazy, they don’t want to repeat the same calculations if they can help it. That’s why they use formulas and letters! If they solve one arithmetic problem, like "how long of a ribbon do you need to wrap a cubic box if the length of its edge is 15cm and you need another 10cm to make the bow at the top" then that only answers that one question, but if they replace the numbers with letters, then that formula they create answers every arithmetic question stated like that, but with different numbers.
When teaching kids, "mathematicians are just very lazy people, that’s why they (…)" has been my go-to explanation for many things. It truly works wonders in terms of convincing the child that maths isn’t some Evil Devilish Subject That Is Never Useful