I'm about to buy a desktop computer (only the computer, no monitor), the brand is Dell Vostro, when I click on shop now and adds it to my shopping cart, it runs some kind of scans and says that my system is ready to be built. What?

I didn't know what it was talking about. My system is ready to be built? So is the computer only for people who is building another computer? So it is okay if I buy it? I already have a monitor and I need a more powerful computer, I'm not going to built anything, so can I buy it and just use the computer with the monitor I currently have like normal?

Thanks :D

They are referring to the assembly of the complete, functional, "normal" system that you are about to purchase. I believe it is Dell's marketing gurus attempt to make you feel as if you're getting a system custom built by them just for you and thus differentiating Dell from all their competitors who, just like Dell, have a large warehouse of PC components and may choose to pre-assemble their systems into common configurations so they can be shipped quickly when ordered instead of needing assembly and packaging once an order is placed.

Note, this is just speculation. I do not have firsthand knowledge of Dell's order fulfillment process. I do know I quit buying Dells for my business clients because of all of the pre-loaded crap that came on them that I had to remove before deploying. This "ready to be built" terminology may also be an attempt to distance themselves from such stigma.

Phone Dell if in doubt.

Yes, provided you don't do something like customize it and, for example, select an incompatible video card that only has digital connections when your monitor only supports analog, etc.