Look, by default as i know linux come with 3 partition as default : boot , /, tmp .. and you can add more if you need. and for you question WHY cos it better from many ways : secure (can add custom privilege to each part), safe (safe other partition from being over qouta and crash the system due to free space problems) .. etc ..
about the BETTER layout , as i told you before about % , coz it same situation here. nothing is better if we dont know what you need from. if mysql servers use big /var/ partition , if file server make /home part bigger etc ..