The developer benefits from reaching more people, some of whom are likely to purchase the proprietary license. Or sometimes you dual-license just so that licenses are compatible. Each license has pros and cons for both the developers and the users.

Qt for example, the LGPL means you need to dynamically link to it, and if you ship your own Qt libraries you must provide the source code for it. But if you're a company that writes proprietary software and can't dynamically link, then you can purchase the proprietary license which allows you to do a lot more, but you're compensating the devs for it. And for the Qt devs that's good because either you pay them, or you use it for free but must share your changes with everyone.

For ElasticSearch, that makes it so Amazon can't just patch it up and sell the modified version without sharing what they changed. They wanted to add back a FOSS license to stop the bleed to OpenSearch which many in the FOSS community switched to purely for the license because even separate software should be compatible license-wise if you want a sustainable FOSS project. But the AGPL requires sources merely for being able to talk to it over the network, so Elastic gets the free dev work, or the juicy license payments. The other free licenses achieve similar goals with technical differences that might matter for the user. But as a developer using ElasticSearch maybe you do want to ship your software under the SSPL, so you can pick the SSPL version.

Dual-licensing MIT/GPL for example, you can build proprietary software, or GPL software where you can vendor it in as GPL-only as well, and thus guarantee your user their GPL rights.