From the rules: 4. You MUST own the name - You are not allowed to post the name(s) that you do not own
This is because it's never a good idea to announce a domain *before* you buy it - some people might see that as interest and register it before you. It's also to save people time and also stop potential negative comments affecting the owner in a case of someone else's domain.
However relying on automated appraisal tools is not a good method of ascertaining a domain's worth. They are completely wrong most of the time and will usually 'appraise' a domain for far higher than its real value (if any). You only have to look back at people trying to sell domains for 4 figures after an estibot appraisal and failing to even secure a 2 figure sale.
If you don't know what a domain could be used for do not register it. You haven't got a potential target market in mind or a route to monetisation if you choose to develop.
Domains are only worth something to someone if they can at some point be used to make money or you can sell based on them already earning you money. Potential is the key for an undeveloped fresh registration.