It makes sense if you need much Ram.
DDR3 RDIMMs used from ebay are really Cheap.
And the price difference for the whole system is huge if you buy used.
I payed for my System with a Xeon 2680 V2 (10C20T) 128GB Ram just a little bit more then the asking price for the 9900k alone.
But I bought it almost 2 years ago so the comparable Mainstream CPU at that time was the 4C8T 7700k.
As a student with a limited budget performance per € matters more then performance per watt.
I picked up a Dell T710 with 32 cores and 32GB of DDR3 ECC RAM for $400. I can throw more RAM in it for practically zero cost. It's amazing for batch workloads.
For interactive workloads, there's no need to get anything newer than a 4-series Core iX. The single-threaded performance has not improved much over the last five years. My daily driver is an i5-6500; motherboard and CPU, used, were $120.
I just got a used Thinkpad with an i7. I was eyeballing cheaper stuff with i5. The reason I upgraded is the security problems in CPU's require mitigations that will probably keep slowing them down. I bought a faster core to mitigate those slowdowns a bit. Plus, get security updates for a while longer.
I recommend people buying AMD where they can. Vermaden on Lobsters is a BSD expert. I asked them what hardware runs pretty much all BSD's. Vermaden narrowed it down to a few Thinkpads. All of them on eBay from recyclers had Intels. So, Intel just came with the box.
If building from parts or doing non-BSD, I'd go with an AMD, POWER, or ARM system.
All the ones I saw in good condition on eBay were Intel's. I got a T420 with Core i7 to mitigate potential slowdowns from future CPU vulnerabilities. I've occasionally had to restart it from suspend/resume issues. Otherwise, it's been great.
One more thing: the function key and control key are swapped compared to most laptops. I didn't like that because I'm used to control being far left. duclare told me about a BIOS setting that swaps them back. Everything's fine now. :)
I got an off-warranty Dell Precision from a previous employer. They were cheap and ordere them with the lowest Xeon CPU at the time, so I spent $100 on RAM/CPU to max it out. 6 sticks of 4GB DDR3 and whatever the Xeon equivalant to the i7-950 was, and threw a couple leftover 2TB HD's in there.
Made a great server. But for a workstation it wasn't really that snappy and I just ended up using a 2016 MacBook Pro.
And the price difference for the whole system is huge if you buy used. I payed for my System with a Xeon 2680 V2 (10C20T) 128GB Ram just a little bit more then the asking price for the 9900k alone. But I bought it almost 2 years ago so the comparable Mainstream CPU at that time was the 4C8T 7700k.
As a student with a limited budget performance per € matters more then performance per watt.