Business loans are granted all the time on the basis that a company demonstrating profit or a plan to reach profitability (this scenario is more constrained) should theoretically be able to pay the loans back.

Investment rounds are similar in this way but tend to address the "plan to reach profitability" or "plan to exit" scenarios more directly than a loan might. Patreon's making money, but Patreon needs to stay ahead of current and potential competitors, continue to expand, and continue to grow their positive cash flow. Sure they could keep going without raising investments, but if they run the risk of being disrupted by competing implementations of the same business model (a very real risk), it'd be stupid not to raise money to accelerate their growth.