They assign different SSIDs A common SSID makes it easier to integrate new devices and change WLAN settings, because you only have to remember one network name and one password. The router can only use mesh functions such as band steering or AP steering if there is a shared SSID.
The router should thus ensure an optimal distribution of the WLAN clients over the two frequencies, so that everyone gets the best speed. The common SSID ensures that the clients can switch between the WLAN frequencies or between router and repeater in a very short time. Additional mesh settings are also only possible with a common SSID: only then do repeaters automatically adopt WLAN settings that you have changed in the router menu, for example.
Current routers only use one SSID for both frequency bands. But you can change that in the menu, like here with the Fritzbox.
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Set different SSIDs for the different frequencies, you decide for yourself and not the router which devices should be connected to which WLAN. For example, you can specify that a device that requires a permanently high WLAN speed, such as a streaming PC or an IPTV receiver, should connect to the optimal 5 GHz frequency. Smart home devices, which are more dependent on range than speed, should only work in 2.4 GHz WLAN.
If a device only supports 2.4 GHz, you can set it up faster or sometimes only by connecting it to that frequency. WLAN problems such as low speed or frequent disconnections can often be solved by assigning two different SSIDs. In most cases, however, the cause is not the mesh functions of the router, but rather the fact that the clients do not support them due to outdated drivers.