A publication agreement typically hands over an author's rights directly to the publisher. Publishers would like authors to think that a transfer of "all rights" is necessary. The courts disagree. Case law has established that an author's right in a work constitutes a "bundle" which can be divided and granted separately.
Retaining your rights as an author is important. It ensures you can use your own writings in teaching or as a foundation for future research or publications. Negotiating contract terms or attaching an author addendum to the publisher agreement helps ensure you retain the rights that are yours.
If you sign a publication agreement, you are subject to its terms, even if it prevents you from re-using your own work! Don't give away more than is necessary.