Foreign investors holding US shares are normally subject to a US withholding tax rate of 30% on income from those shares (typically dividends). However, as Switzerland and the US have a double tax treaty, Swiss investors are eligible to a reduced withholding tax rate of 15%. Filling out the W8-BEN allows IB to levy 15% WHT instead of 30%. You should fill it out as soon as you hold US shares/ETFs.
The 15% WHT from the US is non-recoverable (i.e., you cannot claim it back from the US). However, Switzerland allows you to claim these non-recoverable WHT from your Swiss taxes and this is done by filling out the DA-1. Note that your tax office will not process any DA-1 with less than 100 CHF of non-recoverable foreign taxes (i.e., you must have earned at least 667/567 CHF of US dividends before/after WHT).
The form 1042-S reports the dividends you have earned and the WHT you have paid in the US as a foreign investor. You can attach it to your tax declaration in CH, but it is usually not required as your tax office will compute everything from ICTax and the trades/positions you have declared. You must however attach a report showing those trades/positions.