Deactivating a dynamic page or event serves two purposes.
1 - It's a safety feature. We know how easy it is to click on the wrong button or link sometimes, so we're making sure you don't randomly throw things away that shouldn't be trashed. Think of it as an 'undo' option.
Have you noticed how a deactivated page turns gray in the Pages or Events list? That means it's still available, just not viewable. If you mouse over the title again you'll see that 'Delete' is now an option. If you click Delete the page will ask you one last time if you're sure that's what you want to do. When you say yes, the page is permanently removed from Spark.
2 - Deactivation also gives you the ability to 'store' or 'warehouse' a page when you don't want it viewable at a certain time. Let's say you have an annual event that is only promoted six months in advance. You can deactivate the page/event for the rest of year. At the right time, you can go in and update the details for the next date and then reactivate it.
Keep in mind, deactivation is different from making a page or event 'unlisted', which removes it from the navigation, event listings and search, but anyone with the actual URL can still access it. Deactivation completely hides the page.