This answer borrows ideas from Steffen's comment. The answer is it depends, with a sprinkling of random internet people don't know enough about your organization to give a proper answer. But we can give you some points to think about.
As Steffen said: "Firewalls are used to separate things", usually to protect a higher-security group of users and / or devices from a lower-security group.
In a diagram like that, you have servers, desktops, laptops, and mobile devices. The usual assumption is that the servers and desktops are:
- where the sensitive data lives,
- running all the corporate-issue endpoint protection, anti-virus, and monitoring tools,
- only ever connected a clean packet-inspected network with other similarly trusted devices.
When you draw laptops and other mobiles on a diagram like this, the assumption is that these devices may or may not:
- be running proper endpoint protection, anti-virus, and monitoring tools (almost always the case for phones),
- get connected to random coffee-shop or hotel Wifi networks (or even employees' home networks) during which time they A) come in contact with infected machines and B) can browse to infected websites without the corporate packet inspection and monitoring tools there to protect you,
- get infected USB sticks plugged in,
- be easily monitorable / remotely wipable to prevent data being exfiltrated from the building,
TL;DR: You need to have a think about what your organization's security and threat models are. If you decide that the phones and laptops really truly have exactly the same security needs and risk level as the servers and desktops, then you don't need a firewall there. However, in 99% of organizations, devices that are allowed to leave the building are considered to be at higher risk of infection, so it makes sense to have that firewall and only allow through the hosts and ports that the laptop users actually need to access.
The idea is to assume the laptops and phones are infected and reduce the amount of network probing that they can do on your internal network.