What is the difference between $(command) and "$(command)"?

Question

Most of the time I see $(command) used, but I have also seen "$(command)" for example:

if [ "$(command)" ]; then
    stuff
fi

What is the purpose of the quotes in the second example?

Answer 1

The quotes tell the shell to consider the output of command as a whole. Without them, the script will be broken if the command's output happens to contain whitespaces or any of the special characters \ [ * ?.