Are you talking about the Swiss Travel Pass Flex for 8 days in 1 month?
Swiss Travel Pass | SBB
You do not need to buy additional tickets for the 8 days when you activated your travel pass. The Swiss Travel Pass is your ticket, and it is valid on major train lines, buses, local transport, as well as boats. There is a discount on mountain railways and certain cable cars. You need to buy full fare tickets for the other 7 days when your travel pass is not active, and this can become a costly thing.
From my point of you, the Swiss Travel Pass Flex is not the best travel product for you. It is valid 8 days out of 15 days, the other 7 days you would have to purchase full fare tickets.
I strongly suggest buying the Swiss Half Fare Card. With it, there is at least a 50% reduction on all tickets on all 15 days; combined with a Saver Day Pass, it gives you the same flexibility as the Swiss Travel Pass (when activated). And, you do not have to spend money on tickets in advance, you simply buy the tickets as you go and according to the weather situation, easily doable using the SBB Mobile app.
Swiss Half Fare Card – Switzerland at half the price | SBB
Saver Day Pass – book earlier, travel cheaper | SBB