This is actually an important question.
First, they probably don't have to pay back with interest. Large investments, provided by upper tier firms are "cheap" atm. For loans that means very low interest. More commonly in tech world, these are equity investments. Investors get shares. Dividends are paid in theory, but not or on a strict schedule and not if the company can't afford it. Increasingly, not at all. If you can be securitized and fed into the "high finance" system, you get to exist in a much more attractive monetary system.
A more pertinent question is "why do they need investment at all?" Assuming they can operate out of revenue, most answers to this question are controversial, one way or another.
One reason is cashing out founders and early investors. OF is popular now, but it could lose popularity. Founders are currently paper millionaires and selling equity gives them an opportunity to sell shares too. Even if the company itself sells all the shares, just having cash in OF's account is a buffer to risk.
A more amorphous set of reasons is "getting in with the in crowd." An equity investment is also a valuation event. It gives shares a market value. Besides allowing founders to sell shares, it also makes it easier to compensate employees with options. The company can use shares to buy other companies. Etc. All this relies on shares having a market value, and selling shares to an institutional investor is a way of doing this.
There's also good reason to establish a relationship with a financial backer. You'd rather talk to a merchant bank when times are good, revenue is flowing and investors want in, not when the company is struggling. In the future, you might need emergency cash on a short turnaround. You might want growth finance... likely for a network that needs to scale. otherwise, you leave opportunities for the competition. You kind of need an institutional backers to help you IPO, or otherwise interact with the financial sector.
A lot of this is pretty speculative, but an OnlyFans backed by Softbank might find it easier to negotiate terms with standard payments providers. It might have an easier path to IPO, etc.
In the old days, when firms built factories and made widgets, it was always big news when a big firm signed with a big bank. This was presumed to be a long term relationship, with the merchant bank funding the company and selling its bonds, leading major investment rounds when needed. These relationships were the bedrock of capitalism. Japan's economy for example, was entirely structured around merchant banks. "Keiretsu" brands like Mitsubishi & Mitsui were basically just a bunch of companies backed by a single merchant bank.