The big advantage is having all your files in one device that you can carry anywhere. At one place, you can have monitor, mouse and keyboard, and use it as a desktop PC. In another place, you have a laptop shell and can use it as a laptop. In another place, you can have a projector, and take advantage of the large screen.
Laptops generally fail because of things like broken hinges, broken keyboards, broken cooling fans, etc. If those things break, it costs less to replace a laptop shell than to buy a new laptop.
Librem 5 ($799) + NexDock 360 ($269) is probably cheaper than buying a separate smartphone and separate laptop, and because the Librem 5 doesn’t have moving parts and gets lifetime software updates, it will probably last longer than a laptop.
The downside is that the i.MX 8M Quad processor has limited performance. You can’t upgrade the RAM and SSD on the Librem 5 like you can with a good laptop. The microSD card on the Librem 5 is slow and the Librem 5 only provides 32GB of internal storage. 3GB of RAM also limits the functionality of the Librem 5 as a PC. If you don’t need a fast processor, much RAM and much storage space, I can see the advantages of convergence with the Librem 5.
I think this idea will really take off in the future when there are Linux phones with better hardware. Many people would be willing to give up their PC if Purism makes a Linux phone with the RK3588 processor, 16 GB RAM, 512GB Flash storage and a USB 3.1 port that supports 8K video out. I’m hoping that will be Purism’s next phone, instead of one based on the i.MX 8M Plus.