This update mainly affects HomePod users, as it now finally allows you to pair multiple HomePods at once and sync them.
Although this was available in the beta, the lag between pressing play and the music playing has greatly decreased, fixing the one major issue people had with it.
This update also slightly improved the speed of the iPhone X. Although minor, IOS 11.4 has a slightly quicker response time when launching the app switcher and, more importantly, makes Face ID a little bit faster.
Many iPhone users also noticed after the update that their devices had a few gigabytes more storage space. This most likely means Apple either got rid of some pre-existing software that was rendered irrelevant or Apple decided to delete some cookies and general data that serves no purpose.
All in all, iOS 11.4 is only important to users of HomePods and/or people using the iPhone X. I expect to see less and less updates for the iPhone 8 as we saw today, mainly because this years iPhones are getting rid of the home button completely (although the SE 2 is still very much disputed) so there is no reason to modify something that is irrelevant.
Image via: Apple