Why is pattern "command || true" useful?

The reason for this pattern is that maintainer scripts in Debian packages tend to start with set -e, which causes the shell to exit as soon as any command (strictly speaking, pipeline, list or compound command) exits with a non-zero status. This ensures that errors don't accumulate: as soon as something goes wrong, the script aborts.

In cases where a command in the script is allowed to fail, adding || true ensures that the resulting compound command always exits with status zero, so the script doesn't abort. For example, removing a directory shouldn't be a fatal error (preventing a package from being removed); so we'd use

rmdir ... || true

since rmdir doesn't have an option to tell it to ignore errors.

While it does not affect the output of the program just run - it permits the caller to proceed as if all is okay aka affects future logic.

Rephrased: it masks the error status of the previous command.

michael@x071:[/usr/sbin]cat /tmp/false.sh
#!/bin/sh
false

michael@x071:[/usr/sbin]cat /tmp/true.sh 
#!/bin/sh
false || true

michael@x071:[/usr/sbin]sh /tmp/false.sh; echo $?
1
michael@x071:[/usr/sbin]sh /tmp/true.sh; echo $? 
0