For example, suppose that the investment unit of a life insurance company in Hartford needs to pay a real estate developer in Houston for a $100,000,000 share in a large mixed use development. The life insurer can wire the money to the developer's bank, but how will that payment be matched to the correct account for the correct purpose? SWIFT provides a reliable way to do that.
Otherwise, wires of money sit around as clerks try to figure out which payment goes to which account and for what transaction and with what instructions. The old system of telex messages was prone to errors, security lapses, and abuses. Grabbing a few days float on a large sum of money can be very tempting. SWIFT put an end to those kinds of games and many other issues.
In any event, with access denied to SWIFT, the entire Russian economy will suffer as Russian banks have to contrive alternatives and train staff to use them. Give them six months and they may have it up and running -- but with the US and the EU and our allies still trying to gum up the works at every turn by sanctioning any financial intermediaries involved.
"Without SWIFT, payments and the transactions they are attached to would take longer and need more human attention to be processed and would have numerous security and compliance issues and a higher risk of error."