While my HP serves as more of a "mobile desktop/workstation", my Dell's are always by my side. I have various reasons for using two systems, but the most personal reason is I just want to become a better programmer, and forcing myself to operate in two OS's side-by-side has definitely helped me along the way; also I love gadgets and toys and it's fun... what can I say?
Dual Booting a PCIe SSD Is a Pain in the Butt
I've met several other people with the same laptop configuration as me, several with a successful dual boot, but not one of them accomplished this feet without deleting the Windows bootloader multiple times, corrupting their BIOS, and generally having a really bad time.
When I bought my first XPS 13 this Summer, this was my exact intention - for God's sake it's even on the Dell web-blog on how to do it. But, after deleting my Windows bootloader multiple times (thank goodness I made a backup bootable drive), I relented, and stuck with Windows. Specifically I bought this computer because my beastly HP G3 Zbook was not a practical portable notebook, and I was already 3 weeks into my summer intensive courses and didn't have endless time to fiddle with this new notebook to get it just right.
Note: if you do decided to go down the dual boot route, MAKE A BACKUP BOOTABLE IMAGE OF YOUR DRIVE! Otherwise you'll be on the phone with customer service for hours and generally you'll just be really bummed. Safety first!
Windows-Specific Software Programs
Ah, the bane of all Linux-lovers' existences, the Windows-only software programs that you HAVE to have for some reason. For me, this comes down to CAD modeling and Photoshop. I just can't get these on my Linux machine and they aren't something I'm able to do without (thanks a lot, job). Though I do most of my CAD modeling and Slicing and Photoshop projects on my HP, it is nice to have a smaller portable to get work done on-the-go.
As a Tool to Learn New OS's and Computing Tools
The great thing about Linux OS's is that they are free, and because of this, there have been many great people who have developed all flavors and special-applications of Linux. By having a secondary computer, I now have the luxury of keeping my files I need safe on another computer while I try out all the different flavors of Linux. Currently I am just running the standard Ubuntu 18.04 LTS because it supports all the drivers out of the box, but I've heard great things about Mint and I've been really eager to try Kali!
As a computer science student, CU Boulder and CU Denver have really been pushing for students using Linux. It definitely is a great learning tool, especially for Operating Systems.
On more than one occasion I have had the misfortune of a program compiling and running successfully on my Windows machine(s), only to find out after I've submitted it that the program wouldn't compile on the instructor's Linux computer or server. Often this is usually a Unicode issue or a strange Windows library problem (or Visual Studio), but nonetheless it's been much easier to just avoid it.
Currently I probably spend an average of 9 hours a day on a computer during the week for school and work, that's 45 hours a week, 2,340 hours a year! That doesn't even include "fun" computer time.
Considering these systems will last me about 3 years a piece at their current performance, they were well worth the investment. I was also able to get all three certified refurbished and with an applied student discount which took the sting out of it (a little).
I would be lying if I said I didn't genuinely get excited to mess around with my systems. I get great joy out of scripting a new function on my Linux machine, or getting Windows to do anything the first time through the terminal. It has helped me become a better tinkerer and manage my microcontroller systems more efficiently. I don't have a gaming system, THIS IS my gaming system. And when it really comes down to it, I just love learning about computers, and that's probably the best darn reason for collecting them as I have.