- having a second machine (even an old one) can be nice for redundancy
- the desktop can have more internal storage
- desktops don't (usually) get dropped, lost, or stolen, since they mostly stay in one place
- desktops normally support more external monitors than laptops
- you can use the desktop as a backup target for your laptop (and vice-versa)
- desktops have PCIe slots, which are useful for more than just GPUs
That said, if your needs are limited, it's probably fine. But a high-end laptop is usually more expensive and less performant than a high-end desktop plus a basic laptop.