'laisser-passer' or 'carnet de passage'
 

I'm planning a trip of 8 months trough northern and western Africa and the Middle East by car. Who can tell me the profit of buying a 'carnet de passage' infact of buying a 'laisser-passer' for every country?

A Carnet is just handy, you don't have to worry about finding customs when your laiser passer needs prolonging. (e.g. in Mali after 8 days, again after 3 weeks or so, ...)

Don't know about middle east, maybe you just need it to get in somewhere.