In the game 20,000 Light-Years Into Space you can build nodes, that are not over a steam-source. Does it make sense to build some to strengthen the network or should I only build nodes over steam, to get the most influx?
Wow! I'd never expect to see a question about this game! :)
Yes, it does make sense to build nodes. As you say, they indeed increase your network strength.
The time it takes to (re)build a pipe depends on how long it is. It's very quick for short pipes, but it takes a lot of time to build a long pipe (I don't remember if the upgrade time is also affected). Since the game hazards (earthquakes, aliens, sandstorms) will destroy some of your pipes, it's very important to be able to rebuild them very fast. By breaking up a huge pipe in many smaller pieces you reduce the damage area and also reduce the time to fix it.
In addition, when you do that, you can easily create a mesh made of many redundant pipes, creating multiple routes for steam. This redundancy has a double effect: it makes your network safer from the hazards, and it also increases the overall steam throughput.
Finally, just for completeness sake, the instructions page also says that nodes store a small amount of steam.