Do you have a dual band wifi router? Does it claim to automatically switch your devices to the best band for performance? Unfortunately, the algorithm used by most routers prioritizes signal connection strength overall. This means that while your 5Ghz band might have a usable connection and could provide blazing speeds, if your 2.4Ghz channel is much stronger, your device will be joined to the stronger band even if is slower. And if your 2.4Ghz and 5Ghz both use the same name, you can't just toggle to switch back to the faster 5Ghz band. If you use a separate name for your 5Ghz network and join that network, then your device will stay on that band unless it becomes very, very weak, at which point your device will likely switch to the 2.4Ghz.
Basically, using two names allows to you manually determine which network you devices connect to.
** Edited to add labels **
What you say 10yearfido is absolutely true but I'd just like to mention that if you have some devices on your network that aren't 5Ghz compatible (like a lot of printers or scanners and such) doing this will force you to manually switch networks to the 2.4Ghz one if you want to access those devices from your mobile.
Just something to consider when setting up your networks.
Great tip and thanks!