The traditional approach, also referred to as "the Waterfall Model" is often used when talking about Agile. The waterfall method is executed in stages where the project starts at the top and flows through the different stages down to the bottom. This method has many drawbacks and especially compared to Agile methodologies it also introduces some unwanted risks in the planning process like:
Slow iteration: Increase how often you execute and collect samples for comparison. This makes it easier to detect errors and make necessary changes underway.
Low transparency: Your lacking insight into the process and the changes that are being made inhibits you from controlling the outcome.
Slow ROI: Delivering slowly makes it harder to reap the benefits. Deliver often and you'll receive faster and be able to adapt more quickly to changes.
Silo mentality: Slow processes in your organization leads to siloing of knowledge and employees within their department. Agile ensure that information and knowledge is shared often and freely.
Work in an empirical manner to better predict costs, gain better understanding of the user/organization's values and get better at what you do. Having a framework around your process is an absolute must to be effective, and Agile is the best method out there.
Gather domain knowledge in the form of context, user stories and use-cases.
Split big tasks into smaller chunks. Identify subtasks and estimate work.
Repeat 2nd step for all services in environment until mapping is complete and configuration has been declared.
Workers picks and executes stories and implements according to specification until the iteration end.
The process criticized by everyone who attended. This knowledge is used to refine the process before it’s repeated again.
The entire iteration starts again from the beginning. Using what we learned in Step 5 when we start Step 1 again.