You don´t need a 1st Class pass for Nighttrains as for example on Nightjet you can reserve even a sleeper deluxe with a 2nd class pass :) 

1. of all it´s often less crowded (few exceptions are possible)
2. You have with a 1st class pass acccess to the waiting lounges of Austrian Rail and Hungarian Rail (Budapest only), Czech Republic you can visit them even with a 2nd Class pass and a reservation for a Fasttrain Germany no access for Railpassuser :/
3. You have a bit more of service for example a on seat service from the dinning car on major trains 
4. SwissRail have some special Panoramic coaches on Eurocitý´s from Graz to Zürich EC 163/164 “Transalpin”  that can be used with a 1st Class pass They are even on Route between Hamburg - Cologne -  Rhinevalley - Mannheim - Basel  - Zürich Here some pictures of these Eurocity Panorama cars

Reservations for the Panoramic trains as GlacierExpress or BeninaExpress can be a bit more expensive as in 2nd Class but with a 1st class pass you have always the choice between both travel classes :)
 

Wow, those are awesome pics from the panoramic train cabins. So if I get a 2nd class ticket, can I book 1st class seats with upgrade fees?

Yes it´s usually possible to buy upgrade tickets from 2nd Class to 1st Class
It´s usually the difference between the standard fare 2nd Class and 1st class (Depends on the Route)
I used often Promotional and Saver Tickets in German called “Sparpreis” or “SparSchiene” as they were cheaper as the upgrade and i saved a travelday of my flexipass for another nice stop :)

Makes sense.  Thx.

It’s not a reply but more a supplementary question.

If I do take a 1st class pass, am I obligated always to pay the 1st class reservations etc or can I pay for a 2nd class resrevation and “only” sit in second class?

TIA 

Steve

No you can still decide to book / sit in the second class Depending feom Country there is sometimes no price difference between 1st Class reservations and 2nd class reservations :)