I would imagine in many ways it’s like how a regular development team is different from an Agile development team.
Traditionally, you’d define a project, plan it out and work towards it. The problem with that approach is that when coding software, you don’t always know upfront what you want or need. Nor does the customer, by the way, be they internal or external.
You inevitably end up backtracking or redoing your planning, causing delays.
Modern marketing is similar in that you can rarely get away with planning out a multi-month project anymore. As you deploy parts of your strategy, you’ll get feedback from “the real world”. Things you thought would work, actually don’t. Things you didn’t even think of, actually do.
The Agile approach to marketing, just like in software development, allows you to shift based on the feedback you’re getting. It allows you to change your priorities on the fly, redirect and avoid backtracking.
I’m a big fan because it allows marketing teams to try stuff and fail, without it having long-lasting repercussions. And since marketing is still an art, figuring out what works is paramount.