Hmm, your question "given this scenario, what is the advantages of buying the Dell with Linux pre-installed." really doesn't make any sense to me at all. The closest question that would make sense to me would be something like "Why on earth would anyone buy anything else other than a Dell with a linux pre-installed".
Assuming we're not talking about "Damn Vulnerable Linux" which was written as a teaching aid then the only sensible answers to that would be things like "Because i wanted a toaster" and answers like that. As linux is already the main OS on small devices like watches and stuff i would guess the toaster is more likely to have linux than be made by Dell Lol
Anyway, thanks for this question. It's been fun
Good luck and i hope you enjoy linux as much as i have so far
Many regards to all from