You can start the other way around asking the question. What is it that needs to happen for a computer to get net access?
A typical set of issues needs resolving.
Who is the new computer? All network devices need an address. Since your computer is new to the network, something must allocate an address, and must communicate that address to the computer. This communication of address will typically occur over the network, so there is a chicken and egg issue to overcome.
How does the new computer know how to talk to the rest of the Internet? I needs to know how to find its way out of the network it has joined to the Internet, and it needs to know how to turn network names into addresses. The facilities on the network it has joined that do this are routers, and DNS servers. Your computer needs to know who these are. Someone has to tell it.
The typical answer to both the above questions is a service known as DHCP. The scenario is basically this:
Your computer shouts down the network, “Hi I’m <MAC Address>, what’s my IP address, and stuff I need to know?” and the DHCP server replies with an address, routing, and DNS server info. A typical WiFi network does this as a matter of course.
However a wired network is often designed much more securely. The DHCP server may be configured to only allocate a given IP address to a specific MAC address, and to ignore any other computer. (As noted above this isn’t terribly secure security, but is a speed hump.)
A very typical setup is to have the wired network quite secure, and the WiFi network quite separate, and a firewall between. WiFi connected computers might be allowed access to the wired side over a VPN, but are usually treated as untrusted, and no better than machines on the general Internet. As noted above, different companies have widely differing policies, but typically the idea of a random computer accessing the internal network is viewed badly. Many companies will require that the computer is managed by their IT group, sometimes even prohibiting the end user from having admin rights. And so it goes.