Decision for a need is taken on information+data+rule+solution known to the person/organizations taking decision.
In your question, NEED is "setting up a network"
One of rule known in this question is Network topology, and it is pillar rule for setting up any network that is required in the question.
Network topology affect can be seen in terms of:
1. no. of user support required and how they should be connected
2. What type of multi-level security is supported
3. Type of network device required
4. whether mixed network topology is required or not based on type and usage of network
5. selection of network topology also impacts CAPEX because of the price of network device
look at http://en.wikipedia.org/wiki/Network_topology for more information