Payments processing - a lot of employers will not want to deal with processing payments for contractors. Indeed, its often the main reason why they might go for contract vs permanent resource in the first place. I've seen situations where contractor and employer have discovered each other, only for the employer to then ask the contractor to 'go through agency X'. No one likes admin load and we'd all get rid of it if we can.
Reduce assessment load - job opportunities cycle much more frequently on the contract market - typically 3-6 months. This means a lot more time involved in opportunity sourcing / vetting, potentially a hugely time consuming exercise. A good recruiter will be able to filter these opportunities for you, and only get you the most suitable gigs
Reduce downtime - going without agencies entirely means relying on your own market gravity as a developer of renown to secure job opportunities. This is do-able for high profile developers, of course, especially those who live in metropolitan areas and are prominent on the open source / community / events scene. However, if you work on proprietary software, have heavy family obligations and live outside of a big city, you're probably going to find agents very useful indeed.
Salary / Rate negotations - they are going to take their 15-20%. But they may end up earning you more by negotiating hard with the end employer. Certainly an inexperienced contractor is at risk of being exploited, but that's true in whatever of the type of contract you sign. A good relationship with a trusted agent can really help you make more on your rate, especially if you are not a naturally comfortable at negotiating.
And I say all this as a maker of a tech hiring platform that doesn't allow 3rd party recruiters on it. They have their place - just a smaller one than they currently occupy.
Great article in all other areas
The payments thing is important, and sometimes recruiters provide a useful buffer too - the end-client company might take their sweet time to pay, or be unreliable. Contracting through a third party provides you with a buffer and I know for a fact that I've been paid well ahead of the recruiter being paid on several occasions.
It's a brilliant platform & I've hired a few great people on there.