The reason given for And Another Thing... being written was a much-quoted (in press releases and interviews) mention by Douglas Adams that:
I suspect at some point in the future I will write a sixth Hitchhiker book ... I would love to finish Hitchhiker on a slightly more upbeat note. Five seems to be a wrong kind of number, six is a better kind of number.
And Another Thing... does not really add anything to the existing books. While quite a readable story, it is distinctly non-Adamsian in turn of phrase and structure, which was always part of the enjoyment in reading a H2G2 book. It is in effect merely a different author's book, written in the H2G2 universe, which to be honest was the only way Colfer could have come out of writing this book without a fan backlash.
Overall, it's a perfectly adequate and readable book, but it doesn't add anything to the previous stories and is largely self-contained.