There are plenty of reasons to rework a website: tired front-end design, insufficient design for growth, addition of new interactive features...
When the choice is made to more-or-less abandon an old site structure for a new one, the content within the site may very well be salvagable. In fact, it is worthwhile -- and this is something for the SEO gurus to weigh in on -- to retain some of that content to smooth the transition, give some context to an otherwise substantial change in site architecture.
It's good to give due consideration to what content is and is not worth migrating in the scoping phase of the project. This could be photography, some of which is not dated in appearance and is still representative of your business. It could be verbal content; 'about us' and descriptive language. It could be very valuable blog history.
Depending on how much time/effort/forethought/money was put into the existing site, there is a trove that needs to be gone through and reevaluated. The goal isn't to be sentimental, but to find value in what has been done, give some continuity to the migration, and quite possibly save time in the building of the new site.
And if your site has any product-catalog or eCommerce components, there is almost certainly a dataset of great importance. No one wants to re-key 100's of product numbers and descriptions if there are tables that can be exported and reworked. Being able to identify and provide these databases early on can make a big difference in the efficiency of the project overall. Even if the website has never had eCommerce before (and is now getting it), the time saved in identifying usable spreadsheets or ordered lists and making use of those assets cannot be understated.
Bring that data, and don't hold back; it's easier to trim the fat than to capture the boar.