Data show that roughly 70 million workers participate in an employer-sponsored 401(k) plan as a key retirement savings vehicle, and 401(k) matches offered by many employers help boost those savings each year.

A Vanguard How America Saves report estimates the average employer match in the United States at about 4.6% of pay, in addition to employees' own contributions.

For someone earning $75,000 a year, that amounts to roughly $3,450 in additional tax-advantaged retirement savings annually. Some workers consider this effectively "free money" on top of their regular pay.

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