The really big advantage of notes, especially START-fund style "we'll defer even pricing the note" notes, is that it reduces the number of things an early stage team needs to think/worry/stress about.
Creating a new business is inherently chaotic and stressful. Nothing is nailed down, and some decisions are lasting. I'm no better than top lawyers or salespeople at negotiating terms, and I'd rather not burn my resources on trying to win in a potentially adversarial negotiation with investors over price and other terms early on -- focus should be 100% on product, sales to early customers, and team building.
Any investor who won't let me avoid the hassle of pricing a round, paying lawyers a bunch of money, etc., can sit and do whatever it is VCs and angels do with their spare time for a while until there's enough traction and revenue to justify raising yet more money, hire someone to handle sales and development, and then founders can focus on negotiations with pricing and terms.
Of course, an investor who waits like that is going to end up paying a much higher price for the same amount of equity, since some risk will have been eliminated, other investors will be interested, and money will be less value-add (in a seed round who provides the money matters more than the terms or amount; as the venture grows, terms and valuation matter more and more, culminating in bridge/mezz or IPO where the terms are all that matter.