There's a lot of daft DNS code out there that makes strange assumptions. My favorite example isn't quite relevant here but I'll mention it anyway because I think it paints the picture well: PowerDNS purportedly added compression support in its responses because a common stub resolver required answer owner names to be a compression pointer to the question.

EDIT: Found a reference:

"Turns out some customers were using a CPE router that thought ‘C0 0C’ was some kind of ‘answer starts here’ marker. And if it did not find that marker, its DNS component would crash. And at that time, PowerDNS did not compress that first response record, so there was no ‘C0 0C’."

-- https://berthub.eu/articles/posts/history-of-powerdns-2003-2...

The random mixed-case patterns are used to reduce the probability that someone evil could send spoofed DNS replies to your queries and have your DNS resolver trust those fake replies.

But if an adversary can send a spoofed reply with the desired domain name at all, I expect that the adversary could read the original request packet, too?

They're racing. The adversary needs to have their spoofed reply arrive first so you'll accept it as genuine. They will most often seek to arrange to reply to a query they guess you've asked, such that their answer arrives after it's asked but before you receive the honest answer.

This is why that transaction ID matters, the honest answer will copy the transaction ID verbatim from your question in the answer, so you get to pick it at random (back when I was a child it might just be a sequential counter) and your adversary has to guess it. But, alas the ID isn't very wide, so they really do have a good chance to just guess it. Hence, let's hide more random bits elsewhere in our queries to get a better chance of foiling the adversary.

How does an adversary guess what you're asking? Well, for one thing they might have chosen the question you're about to ask. When a bad guy's web site has an image at the top with <IMG SRC="http://real.website.example/header.jpg"> doesn't your web browser try to look up real.website.example to go get the image? Very predictable.

No, you can do it blindly. For example, if you have a web page that has an <img> tag pointing to a specific domain, then you can assume the client will perform a DNS lookup, so you can send fake replies blindly, hoping it will match a lookup request that you don't see.

Adding mixed case matching makes it more difficult to make a lucky guess when sending a fake reply blindly.