Behavioral, system design, coding, and problem solving interviews are AND gated. It's not a question of one being more important, but which ordering optimizes the funnel and employee time. There are many more calendar slots belonging to tech ICs than to hiring managers; you can run a higher velocity pipeline if you start with a technical phone screen.
The coding interview itself is an immune system against the surprisingly large population of candidates who have long resumes and can talk a big game but not actually perform. (I didn't believe these people existed either, until I started doing phone screens).
It's also a way to recognize smart people who will do well with whatever they're given, even if they're underutilized or in a different domain at their current job. Tech companies are typically hiring more for this than for tech stack or business domain similarity.