Well, for one, if we assume the reason the government taxes is to pay for the amount of spending it has undertaken, then I would assume we would want to tax in a way that more effectively generates revenue. If we tax a person making 30k a year at 50% we will get a lot less than if we tax someone making 300k a year at 50%.
I think everyone would also agree that $1 for a person making 30k a year means much more to them than to someone making 300k a year. If you believe in the idea of "equal sacrifice" than I would think you would agree with progressive taxation.
Also, looking at it from a somewhat moral point of view. Assuming one was making 300k a year but for one reason or another faces a drop or rise in income. I would say that without knowing which would happen, I would prefer that the benefit go to the least prosporous outcome. Meaning if someones income drops, they should face less of a burden then if there income rises.
Progressive taxation can also act as a way to stabilize the economy, since a drop in income will result in less taxation, or in effect stimulus.
Progressive taxation can also work in a way to make everyone better off (especially if it is combined with things like an earned income tax credit), or pareto improving, since it will work in way that reduces inequality, it will keep economic benefits from flowing to a select few while leaving many others behind. This leaves it up to the economy to grow the pie, while the tax policy can work to achieve a more efficient outcome.
