Money is more fungible than votes. If you live in a very red or very blue state, it's probably more impactful to buy $100 worth of political advertising for your candidate of choice in swing states than it is to actually vote in a nationwide general election. (For what it's worth, I'd bet this is still true if you replace $100 with $1.)
It is like voting, maybe with more steps, but with way more normalizing power because you can be very flexible in how you vote. As opposed to voting where you have to have the ability to actually exercise that right in a very narrow time constraint. And on top of that you're diminishing* the influence of people with much more money, since you're playing the game with the same tools.
Politicians often work in the interests of their donors, not just their voters. It's a way to adjust the system so that the voters are also the donors.
