Because, before minimum wage, we had people being paid the price of a loaf of bread for an entire day's labor. We had people being paid in "company scrip" only spendable in the company's own store at inflated prices, instead of real national currency.
In a society with a functioning safety net, minimum wage wouldn't be necessary. That wouldn't only be better for those who can't work at all. All employees would benefit, and eventually most businesses would as well. Here in USA I'm not sure if such a safety net is possible, but I hear good things about other societies.
Only about 2% of hourly paid workers earned federal minimum wage or less in 2018 in the US [1]. This percentage has been dropping over the past few decades. I think this implies that minimum wage has been calibrated to be low enough that it makes little difference overall.
That's a recipe for significantly increased unemployment which would affect the most vulnerable workers the most.
For most people in a low-paying job, the alternative is no job. If they were able to get higher-paying jobs they would have already done so.