much of it is measurable, and the measurable part gets acted on. that's part of why they give the sponsorship a special link or code with a discount, if people sign up with that link they track it, and probably attribute the revenue/profit from that sign up to that advertising campaign. If more profit is generated from that link than it costs the company for the sponsorship (including the cost for the time of the employees working in marketing), the company continues that advertising campaign. it doesn't measure everything though yes, but is enough to seem likely to me that online advertising campaigns do work
also i would propose that you should spend $100 on advertising (including cost of time reaching out to people etc) to generate $100.10 in profit(not revenue) if the return comes fast enough. you can estimate the opportunity cost of spending that money by seeing what interest rate somebody would loan you money for, if that .10% ROI is more than the interest rate on the money, then it's worth doing, even though it's only $0.10. then if you do need to do something else with the money you can take out that loan.
I guess it might be harder to calculate opportunity cost of your employees time since it might take a while to hire more employees, but you can estimate that based on their hourly salary. also hard to calculate opportunity cost of your brand reputation from doing more advertising. and yeah hard to calculate opportunity cost of your own time but you can just estimate a hourly rate and good enough. most of the math is clear though and companies go on that.
(disclaimer: i am not an expert on any of this)