Why we need expertices while doing design for big network ?

Of course, you need an expertise for creating a design for a large output network.

As a large network is sensitive for use and cannot withold any changes again and again, it should be designed in a manner that even if you make an upgradation in the network it will just require small analytical changes and will not disrupt your production for too long. It is considered to make a design for any large network which supports minor or major changes/ maintainence/ upgradations which a wink of an eye.

Thanks.

Because all the big network complexity increases

Because from the design it will lay the foundation of the Network and future Flexibility.