The main reason people combine nodes, is to later energize the node and get a decent amount of steady vis for a vis-relay network, in all 6 primal aspects.
I should note, merging nodes is SLOW. It all hinges around the nodes bullying smaller nodes, eventually all the vis will be on one node. This is not a lossless process either, some of the vis will be lost.
When a big node bullies a small node, it tries to take one vis of a random aspect from the smaller node. Usually this functions as if you had drained a single vis from the node via a wand, but occasionally this can cause a permanent loss in capacity. It then attempts to add the stolen vis to make itself bigger. The less of that vis type it had at the time of stealing, the more likely it is to actually gain it. For the sake of example, a node with no Terra, steals Terra vis from a smaller node. It has a very strong chance of gaining that aspect. However should a node try and steal Ordo when it already has 50 Ordo, the chance of it gaining more Ordo is very low. (I think 1/50, but i'm not sure.)