why is it necessary to understand the culture of an organisation in
order to create a suitable strategic network plant

Is that a trick question or something else?

Anyway, I would say it is necessary for a company's culture to be
understood for network planning for some of the following reasons:
1. Many companies move their employees to different team sizes.
Usually it is preferred for teams to be working together from nearby
offices. This usually requires for a very good network plan so that
you can accomodate multiple users in the same office at times, etc.
2. Companies might have employees work with laptops or workstations.
Both of these require different kinds of networking support.
3. Companies might use discussion rooms, etc. for having network
programming contests, etc. - network planning should take a look if
the company promotes such events, etc.