Sonotsugipaa

Nah, I'm refering to socially engineering scam ads like the classic "ATTENTION! your Windows has error, click here to fix it."

of which 85% got the uBlock Origin treatment and 4% contain my current public-facing IPv4 address

Followed by disappointment because I'm a disappointment

Yes, caches. Lots of caches.

Blowing off some steam after playing Mass Effect Andromeda without post-launch patches

Better than my septem life, that's for sure

So, the original content is lost forever?

No, but it becomes invisible and inaccessible* as long as the filesystem is mounted over it - see this Stack Exchange question and accepted answer.

The benefits are marginal, for example I can see if a filesystem is mounted by simply typing ll /mnt (ll being an alias of ls -lA) - it comes handy with my system due to how I manage a bunch of virtual machines and their virtual disks, and it's short and easy to type.
Some programs may refuse to write inside inaccessible directories, even if the root user can always modify regular files and directories as long as the filesystem supports it.

It's not a matter of security, it's more of a hint that if I'm trying to create something inside those directories then I'm doing something wrong (like forgetting to mount a filesystem) and "permission denied" errors let me know that I am.

No, directories without anything mounted on them are normal directories - which checks out, since you can mount anything anywhere; unlike Windows volume letters, which only exist when volumes are mounted or detected by the OS.

When you mount a filesystem onto a directory, the OS "replaces" its contents AND permissions with that of the filesystem's root.

Here's an example with my setup (hopefully you're somewhat familiar with Bash and the output of ls -l).

Imagine some random filesystem in /dev/sda1 owned by "user" which only contains a file named "/Hello World.txt":

$ # List permissions of files in /mnt:
$ # note that none of the directories have read, write nor execute permissions
$ ls -la /mnt
drwxr-xr-x   1 root root          168 May 31 23:13 .
drwxr-xr-x   1 root root          128 May 31 23:14 ..
d---------   1 root root            0 Aug  1  2020 a/
d---------   1 root root            0 Feb 11  2022 b/
d---------   1 root root            0 Aug 11  2021 vdisks/

$ # No read permission on a directory => directory entries cannot be listed
$ ls /mnt/a
cannot open directory '/mnt/a': Permission denied

$ sudo mount /dev/sda1 /mnt/a

$ # List again the permissions in /mnt: the root of /dev/sda1
$ # has rwxr-xr-x (or 755) permissions, which override the 000 of /mnt/a ...
$ ls -la /mnt
drwxr-xr-x   1 root root          168 May 31 23:13 .
drwxr-xr-x   1 root root          128 May 31 23:14 ..
drwxr-xr-x   1 root root            0 Aug  1  2020 a/
d---------   1 root root            0 Feb 11  2022 b/
d---------   1 root root            0 Aug 11  2021 vdisks/

$ # ... and its contents can be accessed by the mounted filesystem's owner:
$ ls -la /mnt/a
drwxr-xr-x   1 user user          168 May 31 23:13 .
drwxr-xr-x   1 root root          168 May 31 23:13 ..
-rw-r--r-- 1 user user   0 Jul  4 22:13 'Hello World.txt'

$ find /mnt
/mnt
/mnt/a
/mnt/a/Hello World.txt
find: ‘/mnt/b Permission denied
find: ‘/mnt/vdisks’: Permission denied

Please note that me setting permissions is just extreme pedantry, it's not necessary at all and barely changes anything and if you're still getting familiar with how the Linux VFS and its permissions work you can just ignore all of this.

Adding to what the other comment explained:

I use chown 000 so that regular users fail to access a directory when no filesystem is mounted on it; in practice it never happens, because "regular users" = { me }, but I like being pedantic.

As for /mnt, it is supposed to be a single temp. mountpoint, but I use it as the parent directory of multiple mountpoints some of which are just for temporary use.

I didn't know there was a book adaptation of that movie

I decided to simply create directories within /mnt, chmod 000 them and use them as fixed mountpoints;
for manual temporary mounts I have /mnt/a, /mnt/b, ... /mnt/f, but I never needed to use more than two of them at once.

While this setup doesn't really respect the filesystem hierarchy, I wouldn't have used /mnt at all if I were constrained by its standard purpose since having one available manual mountpoint seems pretty limiting to me.
Then again, I have 3 physical drives with ~ 10 partitions, plus one removable drive with its own dedicated mountpoint...

This is the 7th time I see a picture of a table like this, I guess this type of furniture is gaining traction