A formal design step in your development process can help improve the skills of members of the team.
Design is a skill. Like any skill, it is something that needs to be learned. The team can get better at delivering maintainable, flexible and extensible software if they recognise the importance of the design earlier in their careers.
Sometimes, it is difficult for developers to understand the patterns or practices that are being used by looking at the code of a system. The code is very detailed, and it can be difficult to get a higher-level view - an abstraction - by looking at the code alone. Having a design to refer to can help more junior members of the team learn desirable traits in the structure of a system more quickly.
Using a tool that allows reuse of designs, and structures or patterns that the team has found successful, can aid with the upskilling of the team more quickly.