You'll need it to get paid, pay rent, and set up insurance payments — the sooner you open it, the better.
If your residence permit isn't processed yet, a digital bank (Neon or Yuh) will often accept you with just your lease agreement and passport, while traditional banks sometimes require the permit to already be issued. You can move to a traditional bank later, no rush.
Traditional Swiss banks usually charge a small monthly maintenance fee unless you meet certain conditions (age, minimum balance, student package). Digital banks tend to be free for basic use, charging only for extras (premium physical cards, foreign withdrawals above a certain limit, etc.).