A startup is always constrained by time. Specifically, their runway ($ in the bank until next capital raise). It's key to iterate and validate ideas as quick as possible. It all comes down to feedback loops. Here is where DevOps can help.
From developers writing code and quickly finding out if it's breaking anything (test automation, continuous integration). To developers interacting with Designers and PMs to verify features are working as expected (continuous deployment, ephemeral environments), and finally getting feedback from users and customers as early as possible (feature flags). DevOps underpins it all.
Many engineers focus on the tooling aspect of DevOps. But without Dev there is no Ops. This means creating and maintaining automated test. Architecting applications to be continuously delivered (avoiding dependency hell).
Who does DevOps work? everyone. But, as you grow and can afford more specialised roles, you can invest more into Ops. The goal of these roles is to enable and empower Engineers, not to do the work for them. Creating tooling and managing infrastructure. So teams can effortlessly operate the software they build. But then again, Engineers need to keep evolving how they architect their software to enable higher levels of automation and operation.