I saw that these days laptops don’t have that much of a difference from computers with their CPUs, so why would people buy a computer these days? Why do you own a desktop?

Desktops have several advantages over laptops:

1- No battery. Even when plugged into an outlet, a laptop’s power is coming from the battery. If the battery is entirely defunct than the laptop may not function even when plugged in. This goes back to the old engineering addage "the more parts there are, the more there is to go wrong."
2- Laptops are harder to service/upgrade. Space, heat, and power consumption are all critical factors in a laptop. Most (98%) of desktops can upgrade a video card with no problem in any of the aforementioned 3 areas. Simply because a laptop is compatible with a graphics card does not mean that it will cool the card efficiently enough for reliable operation.
3- Cost! Desktops offer more computing power at a lower cost. This is due to several factors including space, power consumption, cooling capacity, and ease of design. Laptops are also constructed to a higher standard because it is likely that they will be moving around while turned on. Desktops do not require this as most people set it on their desk and never move it unless it needs service.
4- More ports and drives. Desktops come with a DVD/CD drive as a standard and many will come with a second one so you can copy discs easily. Most desktops will have at least 2 USB ports in the front and anywhere from 2-8 in the back. Laptops come with a single disc drive (if it comes with one at all). In addition, laptops will typically have 2-4 USB ports total.
5- Screen quality. With a laptop, the screen attached to the computer is likely the only one you will ever use. While you can attach a monitor, most people don’t do this. Desktop screens are not constrained by the stringent power requirements of a laptop and usually offer higher resolutions, more HD capable models, lower prices, and better response times (which reduces the "ghost" effect).