Because it's better. It's more results oriented kind of work. It gives you more freedom. We could argue for hours why, but better read this awesome book about it.
Our development PROCESS
Initial analysis - this stage covers calls, meetups (if necessary) and brief codebase analysis if we talking about support of an existing project. Development - this stage covers the main development of the feature. We cooperate together with you to get best results as soon as possible.
Staging - at this point both sides will be involved in the process of polishing. We will check all dependencies, fix bugs and improve feature if there are any blockers that were not mentioned before.
Code quality assurance - just before release after your approval we will write an automated test for piece of code we have just written to make sure no one will break it later.
Release - the moment of excitement and feedback from reality. We will monitor closely and will be ready to hotfix in case of emergency.
And then the cycle continues. Usually, we plan for two weeks in advance and have a roadmap for the upcoming month. For bigger projects, we create a separate plan.