The answer I always give is "making time for people".
I've written before and at length about how as a leader the people around you are your most important resource, asset, capability, etc. Find good people, set a clear direction, give them space to be good at what they do, trust them and the results follow. But there is always more to do than can be done, so when you've been in the office for ten hours a day for a week or longer and just want to cut it all back what is the last thing to drop?
Over the years some of the most negative feedback I've received is from colleagues questioning why I spend so much time talking to people. Why spend the time face to face? You're wasting your and their time with something that could be sorted out via email.
This is very true - email refines communication down to the words only and that can be very short and to the point. However, much like boiling vegetables can remove most of the benefit of eating them, it removes most of what matters when talking to someone - the opportunity to create a connection. It might be noticing that someone has a problem that can be solved, asking about their new child, hearing something that frustrates them in the office and gives a better appreciation of the environment in which they are working or one of a million other cues. As a manager you can spot the signs of stress or anxiety and help alleviate them. If I were to think in purely mercenary terms, I can't think of many times I've invested time in a relationship and not seen an extremely favourable rate of return.
My director is very good at this - I know he is very busy, but on the occasions when I need to talk to him in urgency he always makes time for me. Our unspoken pact is that I don't bother him unless it's very important and he respects my judgement for when that is. This is built on the back of a strong relationship of (I believe) mutual respect which comes from - yes - making time to talk and drink coffee together.
Simply giving people time can improve their confidence and effectiveness. By making time for me, my director has learnt he can trust me on a long leash and as a result he's getting a significant return in his investment in terms of my effort. In turn, I feel confident that he respects my ability to make decisions - so I feel empowered to keep making them, which makes me more effective.
Moving away from the cold mathematics of relationship building, the welcoming and humanising effects of making time for people can't be underestimated. Everyone has a story to tell and most rarely get to tell it - especially to someone who is in a position of authority. It's extremely empowering to think that the person in charge knows who you are and cares that you're getting on. If that person is you, it's on you to share the love.
There is a danger, of course. A leader needs to be able to control their time so they can still deliver things and there is too much of a good thing. Far more significantly, if too much time is spent with others and the pressure of individual delivery is mounting it is all too easy to spend time talking while also answering email or doing other work. This is utter poison - if you're going to make time for someone, do it properly. If you're not going to be present then don't bother - it's more demeaning to watch someone write email and not listen than it is to just not meet at all.
There is a lot that can be written about the cold logic of this but it devalues the humanity to define relationship building by what one can get out of the deal. We spent a frightening amount of our waking adult life in work and in my opinion it does a huge disservice to everyone involved if that time is just about raw output - we're not just in work to work. For most of us, it's the source of most of our human contact and community throughout our adult life so it's worth putting in the effort.