K8s signals the team values infra and sees it as an important part of delivering their product.
It's an investment like any other and for some teams it makes sense to set infra on a solid path sooner rather than later.
You can move fast on things like serverless but the limitations of those systems can force architecture decisions that are sub-optimal whilst control of your own infrastructure can allow broader interpretation of problems and enablement of more wholistic solutions.
i.e it opens up building things "the right way" sooner in many cases and that can save a ton of time and frustration.
Also in the case where it's obvious you will outgrow serverless it also saves on a sometimes painful migration effort.