Before HFT spreads were measured in 1/8ths of a dollar. Spreads of a penny (or pennies) matter a lot to your retirement. To the tune of billions a year saved for investors.
Spreads were measured in 1/8ths of a dollar because the stock market was stuck in the stone age. I'm willing to bet that spreads would have tightened substantially after the switch to decimal even without HFT.
Who do you think it was who brought the stock market out of the stone age? That would be the electronic market makers constantly pushing for innovation.
The old guard was perfectly content with eighths and fought as hard as possible to prevent decimalization because they were ripping investors off for billions a year.