Depending on the nature of your business, you may want to require a new deal to be created during lead conversion. To do this, go to Control panel > Lead Settings and click on the settings tab.
The default setting is off, because in some cases, you may decide that although a lead is qualified, actually identifying a deal is not possible right away. In such cases, you must periodically review your database of contacts and companies to see if you can generate any new deals with them.
If you have a large number of new leads, contacts or companies, that may not be practical. You may miss opportunities because leads were converted but deals were not created. In such cases, it is better to force deal creation during lead conversion and hold off the conversion until you are ready to create a deal.