By feeding a node smaller nodes, the node can gain aspects or increase the amount in existing aspects. In this way, you can make a node that has all the aspects you want and in quantities you want to make it a one stop node for recharging your wand. That is just one use.
You can energize a node by putting a stabilizer below the node and turn it on with a redstone signal. Then put a Node Transducer above the node and turn it on with a redstone signal. After a little while, the node will turn in to an energized node. You can then use Vis Relays to transfer vis from the energized node to devices that can use it. (8 block range) For example, you can make a Vis Charge Relay and put that on an arcane crafting table and it will use vis from the energized node to recharge any wand inside the arcane crafting table. Only aspects that were in the node will be transferable to devices. I would suggest reading
http://minecraft.gamepedia.com/Mods/Thaumcraft/Node. The last section talks about energized nodes.
WARNING: Do not break the node stabilizer or transducer or turn off the stabilizer, or the energized node will cause a large explosion. You can turn off the transducer and the energized node will turn back into a regular node after a while but there is a chance of the node being damaged and possibly destroyed. After the energized node has turned back in to a regular node, then you can turn off the stabilizer or break the transducer or stabilizer.
One of the ways I've read about for feeding a node is to use a 7x7x7 area with the largest node in the center of that area and a node in each of the 8 corners. The center node can only feed off of one node at a time so this gives the outer nodes some time to try and recharge so that they don't get eaten up and destroyed as quickly. (um, relatively speaking. it is a slow process) The outer nodes will be far enough away from each other that they won't feed on each other. Only the center node will grow.