Because 2/3 of our GDP is made up of consumer spending. By making your cash worth less tomorrow than it's worth today it incentivizes us to spend. If we don't spend our money the economy grinds to a halt in a hurry.

to give investors a more significant incentive to invest rather than save

money in savings accounts may as well be dead money as far as the economy is concerned

That makes sense. I guess deflation is a huge no-go since that means a savings account is a risk-free profiting investment where stocks etc are not risk-free.