Twitter just removed the 140 limit for DMs but users are still restricted by the limit in tweets. Sometimes you need a few more characters to say what needs to be said. Tweetstorms or multi-tweet messages work but can be confusing to users. Multi-part tweets can be lost in the timeline and creates a nightmare with multi-threaded messages or lost context. Prolixy makes mega sized tweets quick, simple and streamlines the interaction with users.
Unless I'm mistaken, there are a bunch of services like this (twitlonger, etc). Not that that's a reason to stop working on this, of course.
So, does this compose an image and attach it to the tweet then? The page doesn't actually tell me what it does. It's not voodoo, it can't make normal text letters on Twitter fit more in a single message. So I'm assuming it makes an image or links off of Twitter. But it doesn't really say that.