First, it's a convenient way to turn around pipelines inside scripts in situations where you need to feed a command output or pipeline into a read loop but the loop needs to be part of the original script process so it can manipulate variables, etc. Meaningless example of optionally incrementing a counter in a loop based on the output of a pipeline of commands:

    while read LINE; do
        if grep -q xyz <<< "$LINE"; then
            COUNTER=$(( $COUNTER + 1 ))
        fi
    done < <(awk '{print $4,$7}' infile.txt |cut -d, -f3)

The other most common way I use it is to diff a couple of sets of pipelines or even just files that need sorting before comparison:

    diff <(sort -u file1.txt) <(sort -u file2.txt)