Money that just sits in a savings account loses purchasing power over the years due to inflation. This page shows the basic idea, for the details pick your topic below.
Unlike AHV or the pension fund, investing is not something you're obliged to do, it's exactly what the word says: a conscious, personal choice to put your money to work instead of just leaving it sitting there.
Unlike Säule 3a (pillar 3a) or the pension fund, your invested money is in principle sellable at any time, you decide yourself when to access it again.
Only invest money you don't need in the short term, build an emergency fund first (see Calculators and Checklists). If you're already maxing out pillar 3a, you can additionally look into a 3a securities solution, more on that under Taxes.