Tubes give synths a more organic and 3-dimensional sound, not so dry and in your face. And they let you adjust the tonality subtly, without using EQ.
Recently I made some changes to the tubes in a matched pair of Pendulum Audio Quartets that I use for tracking. The Quartet is a channel strip with three independent tube stages: a preamp, 3-band eq, and opto compressor.
Previously I had a pair of vintage Mullards in preamp sections—great for low-mid warmth, a smooth top end, and vibe...but are sometimes a bit dark. So I switched to a RCA Short Gray Plate in one of the preamps—mid-range focused, punchy, with a touch of upper-mid crispness. Sounds neat when overdriven, sort of 1950s-ish. Note these are different from the RCA Long Gray Plate which is a fuller and mellower sound (more low-mids), I have one of these too, but prefer it in the LA2A for giving tone to leads.
I still have early 50s Telefunkens in the EQ and compressor sections—clean, detailed top end, nice low extension, slightly pulled back midrange. So now I can switch between a rich low-mid mullard sound, punchy upper-mid RCA, or bypass the pre section for the clean telefunken sound. And it's still possible to get matched stereo by bypassing the pre section. Lots of options when tracking.
By the way, Christian Whitmore of Pro Audio Tubes is a serious tube-geek who knows every tube that has ever existed, makes great recommendations based on what you’re after, and tracks down pristine specimens.