The reason we have NIL is that the NCAA rules prohibiting athletes from capitalizing on their name, image, and likeness were deemed to have been anti-trust law violations. About all the NCAA can do in this field is to prohibit NIL being used as inducements, but that's virtually impossible to enforce (modern twist on an old joke: what comes first? the commitment or the NIL deal?), so, bag men have essentially be replaced by NIL deals. Now, not only can you stoke your ego by "buying" a player for your school, it's also tax deductible as a business advertising expense (if you do it right), unlike the cash stuffed in those brown paper bags (or McDonald's bags if you're Tennessee).
As to why people do this, some businesses may see endorsements as a valid advertising expense, but most are just doing it to help the school. I do think that the money involved has been wildly overhyped. If you have millions to spend on your team, most folks are going to want their name on something, rather than to just cut a check to an athlete who might hit the transfer portal before the ink is dry on your check.