It´s recommended because it´s a  very popular Route In times before Covid the trains were full with Tourist from Europe and OverSea and they are now comeing slightly back. You dont need one but it could happen that the train is full with tourist groups and can be hard to find a un reserved seat.

I travelled on the Golden Pass line from Spiez to Montreux during Easter and then there were lots of free seats.

As the trains run frequently you have the possibility to, if the train is too crowded, leave the train on a station with a nice view, and catch the next train.

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/

By the way. Try to get a seat on the left side of the train. Then you will heve the nices view on the mountains.

One thing more. When you travel around in Switzerland I recommend you to download the SBB app to use for planning your trips. 

To get the correct journey from Interlaken to Montreux you need to add that the journey must go via Gstaad or any of the other stations in the middle of the route. If you don't do that you will get a faster route that not will go through the Golden Pass.

The journey from Interlaken to Montreux via the Golden Pass is about 3 h and 40 minutes.

And IF you decide to do-wait to do that whilst In Swiss-at any counter, not just the comp concened. Will cost a few CHF

Thanks a lot guys, you're the best