In Serbia, we too have chips in ID cards, but they aren't mandatory.
I'm going to write a bit about what I found out here. I assume that at least some of that could apply to other users of electronic ID cards.
I'd say that first step is to have large amount of information. Smart cards can hold larger amount of information that magnetic cards (or at least I got that idea from somewhere).
Next interesting point is that because smart card itself is "active" you can make relatively easy upgrades without modifying entire infrastructure. For example here we added fully qualified certificates for electronic signatures into ID cards some time after they rolled out. With smart cards, he private doesn't leave the card itself, so it is more secure than other storage mediums. Magnetic strip cards can be copied and data from them removed.
Another interesting thing is that it is easier to hide information your government doesn't want you to know it's there in a smart card. For example, ID cards here have fields for apartment number and floor number as a part of the address, but officially government shouldn't have access to such information and those fields weren't supposed to be in ID cards.
I'd also say that smart card format itself is more accessible than magnetic card. Smart card readers are more common than magnetic card readers, so users who want to use their ID cards for signing documents, registering vehicles and so on could just buy a regular smart card reader instead of a more expensive proprietary one with encryption functionalities which may or may not be safe.
Also, smart cards can be used to encrypt data in Windows and for logging in (but I couldn't figure out how to do the second part). Users of smart ID cards could avoid having to obtain another smart card for those functions.