Suppose the devs have already started building your brand new and revolutionary idea. The product backlog contains never-ever-seen disrupting features, flashy interface — development progresses and that's great. And will be even greater soon.
For that, you just need these X and Y and Z features to be done. To lure users in and make them feel your product is a one-stop-shop. Or simply because your competitor has them.
Well, the dev team has a huge list of so-called 'value-added' features to deliver by a planned date. Seems like there's still nothing to worry about — you want to do this to DELIGHT customers. So, why not to start building every feature you think of, huh?
But that's what actually happens.