Sometimes, it's really hard to choose just one platform. Many of us dual-boot systems on our desktops, be it a combo of Windows and Linux, MacOS and Windows, MacOS and Linux, or maybe even the rare tri-boot with all three. We've seen some manufacturers offering dual-boot mobile devices, almost always Windows and Android tablets, but now Ubuntu has an official dual-boot option with Android.
The option is obviously still in the early works, but Canonical has tried to make it easy for you both on the Android side and the Ubuntu side. The dual-boot option is available with most Android distributions, although it is still limited to the devices that have either official Canonical support or community support. Then, once you're in each system it is easy to reboot to the other platform either using an app.
Of course, Canonical is very clear that you should know what you're doing if you want to test this out, because Ubuntu dual-boot does flash the Android recovery. If you don't know how to fix that in case of an error, you may want to steer clear. Still, Ubuntu Touch is getting closer and closer to its proper release on some official hardware, but in the meantime work is continuing on giving the platform all of the features you may want. Dual-boot definitely falls into that category.