Some people claim that code's worst enemy is its size, and I tend to agree. Yet every day you keep hearing things like
Question: When is "#lines of code" useful?
ps: Note that when such statements are made, the tone is "more is better".
Reminds me of this:
The present letter is a very long one, simply because I had no leisure to make it shorter.
--Blaise Pascal.
I'd agree that taking the total number of lines of code in a project is one way to measure complexity.
It's certainly not the only measure of complexity. For example debugging a 100 line obfuscated Perl script is much different from debugging a 5,000 line Java project with comment templates.
But without looking at the source, you'd usually think more lines of code is more complex, just as you might think a 10MB source tarball is more complex than a 15kb source tarball.
I have found it useful under two conditions:
Gauging my own productivity on my own new project when it's heads down coding time.
When working with a large company and speaking with a manager that really only understands widgets per day.