While branching can be beneficial for collaborative explorations, there are also scenarios in which working in a single file is the way to go.
If you’re starting from scratch or doing a complete ground-up overhaul on an existing design, it likely makes sense to start with a new file or duplicate an existing file. This keeps all new work completely separate in its own file, which can live in any project, team, or your personal drafts.
You may want to simply update your main file if you're making minor updates or have multiple collaborators all making little polish changes. For example, copy edits, layer organization, updating colors to use shared styles, or wiring up a prototype are often done right in the main file.
When you're exploring multiple concepts or collaborating with others where you may need to actively reference each design, using pages in the same file may be more convenient. By using pages, you can essentially have multiple scratchpads by concept or collaborator, allowing you to quickly jump and move items between the pages.
That’s totally cool! Working in Figma, you still have automatic saves and version history where you can revert to or duplicate any previous save points to a new file. You can continue to work and collaborate as you do today.