For one, people fear death and things related to death, partially because it's the greatest most looming unknown. It's a power that overtakes us all and we can't escape it. It causes us to check out at some point, and we have to wonder, is there anything else afterward, and if so, is it good? The dead are a great trope for horror because they represent all of the fears we have related to death, as if contact with the dead will infect us with death and expose us to all that uncertainty and powerlessness.
If we keep in mind that the dead were once living, some of these fears can fall away. They're not some kind of spooky sort of monster, they're people just like us. They're people who live in the great unknown and can come back and give us insight into it.
Another item that comes up...the dead aren't super paranormally wise and knowledgeable. They know what they knew in life. They may strip away some of the elements that held them back in life and be a more authentic version of themselves, but they are still spirits that were people, and they think like people, specifically like the people they were in life. That said, they have access to other spirits, and to information they can find in the underworld, and that they might observe from watching the world of the living. So their knowledge base may be increased, but they aren't necessarily wiser, and they aren't all knowing.
It reminds us that working with ancestors, or other dead, is not some super mystical wigged out experience, but it's still important and useful. One of the Social Magic Reactions shows Harry Potter fighting Voldemort with the spirits of his dead allies with the caption “what working with ancestrals is like.” My response... “dude, you wish you were that cool.” Again, the dead were once the living, they're not magical super heroes.
Still...again...the dead were once the living, so unlike some magical super being, the dead understand what you're going through. If they're you're ancestors they're bound to you by blood, maybe by magic, maybe by love they felt for you in life. They want to help you, they want to be connected to you. So they can be pretty powerful allies within the range of their powers. They are powerful not necessarily because their powers are ranging but because they actively look to intercede for you from a perspective of insight and sympathy, since they know you and what it's like to be you.
Aside from wanting to help you, they can be a source of power and authority. Their blood connects to your blood, and therefore the power that connected to them naturally connects to you. In many traditional systems of magic a link to the dead is a link to power and authority for the magician. In this way, families that have magic share a connection back through their lineage by way of their blood to the power of the past. Initiatory houses share a link by ritually connecting to the spirits. In both cases you draw on the power of those who once were living, working through the same magic you work through now. But now they work a new magic, one in which they can sit in congress with spirits within the world of the spirits, and so in addition to sharing in their power, they add to your authority, leveraging their position in the world of the dead, because of the link inherent in the fact that they, like you, once were living.
In this simple quote, there is a lot we can unpack. What else do you see hidden in there?