There's a bit more to it than that. Let's say you already had your Tunisian PPL and a couple of hundred hours: you could do the ATPL exams and go straight into a CPL course (because it's all dual flying). But if you've got a Tunisian PPL and want to use it to hour build in another country using their aircraft then you need to have your PPL validated for that purpose (or find a Tunisian registered aircraft there!)
Each country is different in that respect. Iceland for example give an automatic 3 month validation.