Physical design – cable lengths, switch closets, that mini router behind a desk to ‘temporarily’ connect the new guy. Failure to document the layout as you build it will double the cost of maintenance and growth.
Logical – imagine trying to explain to your boss why accounting is no longer connecting to the server, because you don’t remember the VLAN setups. If you only have fifty users and one subnet you can keep it in your head, but don’t take any time off. Documentation is how you get the money to cover growth. If you can’t show what you have and how it is being used, why expect to get money when they want more connections next year.