Hello Yaiyuewai53
You must always buy Supersaver Tickets in advance, a week before departure is too late. These tickets may be sold out of more expensive.
If you want regular full fare tickets, it is not necessary to buy them in advance, the price will be always the same.
Except for some international trips, you can purchase reservations separately and you can keep them. You can find more information here:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Kind regards
Danhiel