If you for example use fixtures like chandeliers, you may install a large number of similar lights, all serving the same purpose. Those lights are usually meant to be controlled all at once, too.

This is what groups are for. In WiZ, a group is comprised of two or more devices with similar lighting capabilities. Those devices will always be controlled all at once (you control the group), and they will share similar settings. A group is shown in the interface as a single controllable item, which drastically simplifies your daily usage.
If the exact lighting capabilities of multiple devices in the group differ (CCT ranges), a group will only do the common part of those, what all the products can do.

In the WiZ home structure, groups are nested inside rooms. Thus the structure looks like Home > Room > Group or individual device.

Thinking about a few examples, you may for example want to group:

• All the candles used in a chandelier
• Multiple ceiling panels used to light up the garage
• The two Squires used for mood lighting in your Hi-Fi corner
• Multiple smart plugs...

Lights within a group will always apply the same light mode (or On/off in case of plugs), at the same brightness. However, creating a group preserves the randomization within light modes, so that even if your lights are grouped they will not always all play the same color at the same time. This helps keep things immersive.

In other words, if you set a group of light to "Party" at 60% brightness, all of them will use Party at 60%, however, one may very much be in blue while the other one is in yellow.

Note that on-board buttons for products in a group will control the whole group.