I've seen people spend days and days trying to save a few hours of paper design time.
We often do quick paper or marker board designs, which often have some UML. You don't want to spend more than 10% of your time on this, but it can save quite a bit of time to market. If someone really wants the design saved somewhere, we just take a pic and upload it.
I think you are right and it depends so much on what you are building and in what context.
If shipping is expensive, you build something complex and correctness matters, you want to go all in on design and verification. You might even want to use TLA+ to verify your algorithms beforehand.
If you build a SaaS app that people use for leisure and is mostly CRUD, you probably don't need UML diagrams for the vast majority of your work. Exceptions will be the larger systems design where you still need to plan things like DB sharding approach, services etc.
I've seen people spend days and days trying to save a few hours of paper design time.
We often do quick paper or marker board designs, which often have some UML. You don't want to spend more than 10% of your time on this, but it can save quite a bit of time to market. If someone really wants the design saved somewhere, we just take a pic and upload it.