Almost every time I ask this question (online or in person), the answer is mostly 'marketing'. I've come to interpret a lot of that as 'this founder can now say they've raised an $x round' and also now the founder's share of the company is with $y.

I just don't know why anyone would want to dilute their cap table so early on, before the product is even public, unless their intention is to exit before building a decent company.