Carousel, like Spotify and several other apps, use storage space on your phone to decrease the amount of data that must be transferred while using the app, thereby saving money and improving the user experience. Different apps allow different levels of cached data management. For example, Tumblr allows you to "smash" the cache, deleting it entirely, whereas Spotify has no way for the user to manually remove the app's cache without deleting the app and reinstalling, a workaround at best. Spotify would like their users to simply trust that their supposedly intelligent data management algorithms will automatically make space available when storage is getting low. However, these algorithms are rather selfishly inward looking, as far as I can tell. The cache is only reduced when that app is attempting to write data and can't find the space available. It's like your roommate taking up space in the fridge and then only throwing their moldy leftovers away when they need room for more of their own groceries. No awareness of the needs of others! It's a technical limitation, but still quite rude behavior. Carousel has anticipated this and employed a clever technique of placating the user. I have an idea of how selfish they're being, but I still felt a bit better about it after seeing this screen used to manipulate us into complacency.
