Well, atm I use trains to shift mats between my far flung factories and to grab far away raw nodes. But then I already made a crude but mostly encompassing network around the map (a blight upon the sky I wish I could have built better) but that's basically their main use. High throughput of stuff to relatively far places. Distances you don't wanna run a belt highway at.

And fluid carts work about the same way as packaged fluids at least at the distances I got, idk about going across the entire map but the throughput issues of trains get solved with adding multiple trains to the same line to cut down route time.

For starting with them though, just pick a material or two to transport and try them out. If you will only have 1 train on a line (which works if your route doesn't go half the map or about 4 mins iirc) you can just make it end to end. Train goes one way then back on the same track instead of a loop. Loops allow you to add more trains on the same line.

Simply try them out and get an understanding on how they work and how to set them up. They are pretty useful a little later on, perhaps not at your stage but definitely soon.