So with Hue you need the bridge for a few reasons. One, to relay instructions to the bulbs over the Zigbee mesh network that the bulbs create. To HomeKit, you can think of your bulbs as a single device with many functions. It also means that only the hub needs network access, and that there’s only one device added to your network. Wire in the hub (I don’t know if the Hue hub has wi-fi or just Ethernet) and you preserve the performance of your wireless devices.
The zigbee hubs, like the hue hub, will usually support around a hundred bulbs or other devices before you run in to any issues.
With any devices connected directly to your wi-fi you hit the limits of the router (generally around 50 connected devices) and you add to the contention on the network, which will impact your speed. Also each device is a potential vulnerability.
Zigbee networks shouldn’t interfere with your wi-fi, generally, but potentially could interfere with other Zigbee installations if you live in apartments or suchlike. Also because Zigbee is a mesh network, each device extends its effective range, meaning that devices can be more power efficient and also helping to prevent dead-spots.