Ini mobilku : This is my car. Inilah mobilku : the -lah here just functions as some kind of emphasis, "This is MY car". Like proudly showing it off to people. Ini mobil punyaku: This car is mine. Pretty much the same as the first one.

The meaning all of them is "this is my car" It's talking about ownership But Indonesia is one of country with so many different language and culture. So It's just about habit in speaking Sorry my english not too good. Hopefully you understand what i mean

when you want to show off to someone or surprise someone you can use "-lah". "Inilah anakku, yang kemarin juara," "inilah mobil baruku."