I would like to add something to Michel's answer, there are two 'synchronization types'.
Hi HP,
I think in general you need to synchronize the modeler file whenever you've made changes in any of your metamodels. The console will check if your database structure is still in sync with your model and if not asks you if you wish to synchronize these changes.
Emptying your cache is only necessary if you've made changes in forms of styling, forms can be cached and if the cache is not emptied it might look like the changes you've made had no effect