Think of them like old, proven architectures of yore that run virtualization stacks with APIs with the same kind of capabilities of Docker and you'll understand how awesome they could be if we as an industry put more effort into adopting them as platforms (sans enterprise BS vendors though). You can run Windows and Linux on modern mainframes as well, so they're handy there as well. Add in features that are very, very mature (like, since the 80s) like hotswap CPUs and memory that VMware has added a few years ago themselves. If you're in a typical F500 with terrible, fragmented IT management, mainframes are a breath of fresh air practically because there aren't roving bands of sales goons trying to sell you more management software driving wedges between different silos of dysfunction.
If you'd like to view them more from a hardware deployment perspective, you can think of them more like the Dell Poweredge VRTX series than your usual decoupled stacks of network, pizza box servers, and SAN chassis. Cisco UCS is more connection-oriented with the emphasis upon making your network topologies far more flexible than with mainframes, so mainframes are compute-oriented versions in that respect with historically weaker support for scaling out with interoperability across the rest of your network infrastructure (not sure if anyone at IBM or CA has seriously taken standards like BGP and OSPF into account for mainframes, that is). People still remote into mainframes with TN3270 commands, I know we had customers still asking for that as of 7 years ago with no expectation of sunsetting.
If you'd like to view them more from a hardware deployment perspective, you can think of them more like the Dell Poweredge VRTX series than your usual decoupled stacks of network, pizza box servers, and SAN chassis. Cisco UCS is more connection-oriented with the emphasis upon making your network topologies far more flexible than with mainframes, so mainframes are compute-oriented versions in that respect with historically weaker support for scaling out with interoperability across the rest of your network infrastructure (not sure if anyone at IBM or CA has seriously taken standards like BGP and OSPF into account for mainframes, that is). People still remote into mainframes with TN3270 commands, I know we had customers still asking for that as of 7 years ago with no expectation of sunsetting.