You can spend the 8hrs how you want depending on the station you may visit a Lounge of the Railcompany if you hold a 1st class pass. Or you explore the City/Village.

Can i ask you where you should wait 8hrs for train. Usually it´s not more than 2hrs.

I was looking at train times from Nice to Barcelona and in Barcelona, there is a wait time for 8 hours. But that is the only one I saw so maybe I just need to do more looking because I think you’re right, for the most part its only like 2 hours. 

Better way: split up in parts-check from N to B and again B to onward, this will mostly show more trains. 8 hrs usually means from last train/day till 1st/morning-which should be blindly obvious by checking times=when normal sensible people take a hotel. Lounges are few and far on railways and several will not take passholders-only their own estimated expensive beeznezzaccount holders.

Its also a good idea to play around with the planners (preferably use bahn.com) to see what other options there are-this app is the last to trust/use.

Thanks for the responses! What if I am traveling overnight and have a 2hr change train time, do I just wait on the platform for the 2 hours in the middle of the night or is the station open or something like that?

The station itself will be open but i assume the shops will be closed :/
Looks like a change in Villach? The Nighttrains arrive quite early and have a long standing time in Villach as they will be splitted and rebuid new trains :)

For example the Nighttrains from Milan & Rome arrive almost the same time in Villach. Each train have carriages for Vienna & Munich In Villach they will split the carriages and build “new trains” with all the carriages for Munich and one trains with all the carriages for Vienna :)

For which date and departure time? In Spain, the timetable for more than 2 months into the future may not be complete yet.