Agile practices help you develop software that meets the user needs faster and safer — and that responds quickly to changes in the requirements, environment or technological advances. But, there is one "secret" practice that is not often mentioned in the context of Agile development. This is really an "un-practice". The idea is to flat out not do something. It could be a requirement (this will require negotiating with the customer), a refactoring or a bug fix. Just because something is on the backlog doesn't mean it always needs to be done. Extreme Programming calls it YAGNI (You ain't gonna need it) where you postpone doing things that are not needed immediately.
Being minimalist by design is often neglected. Everybody wants to eventually conquer the world, right? Another aspect of this mindset is over-engineering. A lot of effort is expended towards building infrastructure, scalability and automation that isn't necessarily needed. Why is it so important and why is it often ignored? It is important because Agile is all about delivering real value, really quickly. If you work on something that's not really needed, you just wasted time and effort.
The reason it's often ignored or not practiced fully is that it's difficult to be disciplined. You start working on a cool feature or capability and want to keep evolving and improving it even if it's not providing immediate business value. On the infrastructure/implementation side, developers are often worried about technical debt. I'm often guilty of trying to get the project "right" from the beginning. If you want to really deliver the maximum business value in each iteration, you have to be very aware and explicit about what you plan and how you go about it. Just paying lip service to the idea is not good enough.