Hello Ckiexonnoi91
Holding the Swiss Travel Pass, you do not have to book tickets, the Swiss Travel Pass is your ticket. You do not have to book trains, as our trains have free seating. Read more on that here:
https://community.sbb-tickets.cc/d/23170-purchased-a-sbb-6-day-1st-class-rail-card-cant-reserve-tickets/4
Why are upgrade tickets available, though? Well, in the event you hold a 2nd class travel pass, and you wish to upgrade to 1st class for a specific journey, you have the option to purchase upgrade tickets.
Kind regards,
Roland