One reason for a mock-up is to verify things such as stability. I saw a video where a coffee table with essentially three legs was modified because it was not stable and kept falling over. This was discovered based on the mock-up.
Depending on the design, you might want to do some sort of practice bit on it to check for things like:
- Is your skill up to the task
- is it prone to damage because some portion is very fragile
- and similar.