I submit a few items but by no means a comprehensive list. What the other posters submitted covers it.
My favorite project managers have had some technical footing. So when I say X is hard for this reason, they trust me and don't ask for X to get done by yesterday.
Remove the obstacles for your team that don't require their technical expertise.
Shield your team from unreasonable expectations and scope creep from other stakeholders.
Add a buffer to every estimate. Engineers are notorious for underestimating.