People have RSUs. Those RSUs vest at some point. When they vest, they have to pay taxes on them, at whatever value they have at that moment. This raise allows Stripe's employees to pay those taxes, and gives them a chance to sell if they want to. If they don't sell, they'll have to wait for future liquidity events, like private raises, buybacks from Stripe, or IPO.
The down round is appealing if you're intending to hold the shares until later, because you pay employee taxes at a low valuation, and then capital gains on the difference between that low valuation and your eventual sale price.