SBB has a sleek and very efficient smartphone app that you can use to buy tickets, day passes, check timetable in real time etc.
There are two major advantage of using SBB app over its website: You can pre-register a credit card, which will then be validated with a CHF 0.01 charge (or something like that). Then, your card is tied to your account. There are far fewer instances of refused transactions with the app. Furthermore, the app also sells local public transportation cards in most areas, so if you arrive at Zürich airport (for instance), you just need to type the address of your hotel and the app gives you the route, and allows you to buy the ticket from ZVV (local Zürich transportation operator).
The app stores your tickets as QR-codes (they can still be also sent as email QR codes for redundancy). It gives you real-time notifications of platform changes or detours, should they happen during your trip. QR codes are stored offline, so even if you don't have WiFi or phone sign, your tickets are there, available for easy inspection.
Single-trip tickets (at full price or advanced-discount price - train specific) and Day Saver Pass can be bought there (among others).
Swiss Travel Pass is not available on the app!