What tubes provide that solid state does not is a complex question. For me, at the core, tube gear can provide more of a three dimensional soundstage, a midrange that no solid state gear can replicate, and an illusion that the recording is live with the right setup and recording.
This document provides some details regarding the differences between tubes and solid state:https://spectrum.ieee.org/ns/pdfs/08_98_tubes.pdf
I currently have solid state gear from McIntosh and Devialet, both of which are among the best available from solid state. While they both sound very satisfying, the tube gear I have provides even more enjoyment with a lot of music from the collection on hand. In order to get the "best bang for the buck" with tube gear, decided to go the DIY route. This provides a whole host of advantages:
1) Point to point wiring vs. PCB
2) High quality parts selection (caps, resistors, connectors, etc.)
3) Use New Old Stock (NOS) tubes still readily available at a reasonable cost.
There are some great resources available which allows selection of circuit topologies which allow for the creation of great sound. There are multiple choices for tube amps, be it Single Ended low power, or higher power push/pull in triode, ultra-linear, or pentode. The preamp design I used is a low gain from Thomas Mayer, which I would put up against any commercial offering. The power amps that I came up with use the octal 6BL7 tube for driver and phase splitter, and the 1625 tube for one set of amps, and the 6883B/8552 tube for the other set of amps. These power tubes are very rugged, readily available as NOS tubes, and are very reasonable cost.
Getting the best sound from tube amps is definitely more challenging than solid state, but to me, can be worth the extra effort if done with care.