How & why to create a roadmap?

Ha precisely someone wrote an app for that here:
https://www.indiehackers.com/product/productific/-Li2hzf1r97yKgGexY6Z

I would say, keep it simple, start with a vision of what your product will do on a final stage when it's completely done and then start listing the steps or features to get there (epics).

Don't worry too much for the details of each epic, each one of those steps can be subdivided and/or have their own road map.

They are useful to prioritize work and define what your product will do on each phase. Most of them contain a due date, or development quarter (or year), some of them don't. Also helps to communicate to the team members and users what is happening.

For instance, say you want to create a social network app. What goes first? The advertisement? Create users? Invite friends? Post photos? Create events? Of course your view of the final product includes everything, but you have to organize it in a way you can do it all. So you list all your features

• Create users
• Friends
• Posts
• Events
• Ads

And then define what is important and what not so much and put it in milestones, you can decide to tackle one feature at a time, or do a bit of each in each milestone.

v0.1

• Create account
• Create posts
  v0.2
• Invite friends
• Likes comments
  MVP v1.0
• Scale
• Performance

v1.1

• Add ads
  v1.2
• Create events

etc. So basically all the work you do in a product will belong to one of these epics, those epics are divided in stories and the stories in tasks (task in subtask)

A very simple product would have a very simple roadmap.

BTW notion has a template to create roadmaps: https://www.notion.so/Roadmap-0da980cac031415584861d5fd5d30118