Let's say you have a team of 5 people working on a project, 3 engineers, 1 project manager, a designer (for example's sake). Each of those people will be costing, at a minimum £250 per day. 250*5 = £1,250. If your project overruns by a month because of poor management, that's firstly ~£25,000 of unexpected cost, and also £25,000 worth of resources that could have been allocated elsewhere.
This is an often overlooked aspect of projects is people. A great project manager will be able to assign people to the right jobs in order to get a balance of efficiency but also enough diversity in the work so people don't get burned-out (bored). There is also the skill of sorting out people's personal issues diplomatically so they don't disrupt the team.
As a product manager (a once data scientist, then project manager now head of product) I could list a hundred things. I would recommend reading some books about modern project management, they tend to be quite sort and will give you skills for life that will let you climb up the ladder from a developer into management very quickly (you won't want to stuck in the product development side of things into your thirties).
Rework by David Heinemeier Hansson and Jason Fried and Zero to One by Peter Thiel are good starting points.