IWP is coming and everyone with a Site Builder website can help us out by getting rid of unused or deactivated pages and assets, deleted pages, and unused files and images. There is a significant overhead for the servers to keep track of the content and assets. All of these items take up space on the servers.
On average all Site Builder sites have 1/3 of the content and assets not being used. One change can cause the site to Republish All Text Assets. When this happens on a large site, the server can slow down as it re-inventories all of your content. It can cause the database to be overwhelmed, and crash the server. Removing all the unused stuff can boost the speed of the server and site and help it run more efficiently. This is why it's important to keep your Site Builder sites maintained by doing house keeping on regular basis.
Here's a guide on Site Builder Maintenance.