A lightweight PC small enough to slip in a folder was what they wanted.
The Macbook Pro was generally for more of the processing-intensive work and development side. Recently I've been seeing/hearing more and more dissatisfaction from that crowd.
In an era of remote servers and web scripting languages, not every developer needs a full-blown workstation.
I agree it wasn't indended for developers, but it ended up being adopted by many of them.
Doing front end development, it was more than powerful enough for anything I had to throw at it.
I haven't really found that to be the case, especially these days with "modern" frontend tooling, like Babel/Webpack/etc, static analyzers like ESLint/Flow/Typescript, and the various types of frontend testing. Every bit of performance helps.
You can debate whether all of that is necessary, but the reality is many companies use them.
It was with the second generation (2011) that they knocked it out of the park, where they added fast Core i5/i7 CPUs, SSD, greatly improved battery life, two (!) USB ports, choice between 11' and 13' models.
And most of all they sold it at an entry level price (for Apple standards). Considered that the competition at the time were bulky Sony Vaio, HP and Dell "bricks", it was like a laptop from the future.