Well, there can be many reasons why you would want to construct a custom home. The generic reason is that you can design it the way you want and can get all the amenities that you want. The same cannot be said about a production home. You would have to compromise on a lot of things. The design of the house would be mostly fixed by the builder and you can only ask for very little insignificant changes. Also, you cannot get additional amenities which you wish for.
One great benefit of going for a custom home is that you can future-proof your home. You need not think about changing your house for years to come. But, in case of a production home, you might want to look for a change after a few years due to dissatisfaction.
With a custom home, you can focus on building a truly energy efficient home. You can find builders who can help you with that. However, you cannot always build a completely energy efficient production home. There are lots of factors which prevent a production home to be truly energy efficient.