In my experience, a portfolio does have benefits over just a blurb and contact details, especially if you don't have a strong network already in place, but it's a question of content and what you're trying to get across.
Yes, if you have a strong reputation in the wider community then it might not be needed but 99% of designers and developers don't so a properly structured portfolio that explains your method and the outcomes can only help. If nothing else it forces you to crystalise your work in your own head and have the right language to talk about it when you get the meetings.
Side-point: Don't waste too much time scouring the web for job posts and waiting for clients to post them. They don't post most of their openings unless their network options are exhausted. Get yourself in their network. Email companies you want to work with and introduce yourself. And get a strong LinkedIn profile, believe it or not, it's not all recruiters on there ;)
While focusing on other things rather than your portfolio (like having a decent proposal and mentioning the value you can add for their business) is definitely a good advice, killing your portfolio isn't really a good one. You can still talk about how you're going to increase conversion rate with an online portfolio, and in fact, having proper case studies online increases your chance to be discovered by clients "out of nowhere".
I've worked as a freelancer for 2 years without a decent online portfolio (because of NDAs and clients not able to raise funds to launch the product), and it was such a pain to send them a 55mb pdf all the time. Then I updated my website and had 240% revenue increase in a year. Obviously, these things work differently for everyone.
I’ve met design firms doing millions of dollars in client work with nothing more than ”coming soon” on their website.
I've noticed this too! So many big agencies' websites aren't that great, and still manage to find great clients. How do they do it?
Yep! Jonathan Stark talks about this a lot and has some great insights. People like this go into business relying solely on their network to get them referrals. This can be great but usually only lasts a few years. Most people need a way to find new opportunities. Finding projects for outbound is something designers don’t like to talk about. Building a better inbound generating system through positioning is another.