2g vs 5g wifi - Why two SSIDs?

There are separate SSIDs for 2g wifi vs 5g wifi, this is different from my previous routers which used same SSID for both. Are the 2g and 5g completely different networks which separate traffic between the two bands, e.g. two separate home networks vs one combined 2g/5g network? Confused.
Comments
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Hello @JamesW
The biggest differenece between the two band is speed as mentioned here.
For instance, if you are using the same SSID for both band, sometimes you will notice that the dual band device falling into the crowded 2.4GHz band, for whatever reason. By using a dedicated SSID for the 5 GHz band, you can force your device to stay in that band.
The benefit of using different SSIDs on 2.4 and 5GHz is that you can explicitly control which devices use 2.4 and which use 5GHz by selecting the desired SSID on each device.