... I mean, if you render a track, it's your computer that renders it, not your external soundcard.. so either way the track will sound like it's made without an external soundcard..
or do you guys record it through the ext.card to another computer/system?

It's about having accurate sound to record/mix/master with ... if you can hear what you're mixing, you should (in theory) mix better. A good audio interface is essential imho.

That's correct, your external soundcard plays no part in the final render. However, when you're in the process of making your music, latency comes into play. External cards have ASIO drivers, which means that the delay between you doing something (inputting a note, twiddling a knob) and hearing it is greatly reduced to the point of being invisible. It's just an annoyance if you're solely inputting data with the mouse, but it's a major problem if you're trying to record parts in live or playing around with ideas on a MIDI keyboard. Programs like ASIO4ALL can help get low latency with your built in soundcard, but it'll never be as low as an soundcard with a good ASIO driver, and (if I understand this correctly) you should be able to get the same latency as a built in soundcard on an external with much less CPU usage.

Then there's straight sound quality issues. Take the noise floor on my laptop for instance. Plug speakers into the jack on my laptop and you hear the hiss straight away. The noise floor on my laptop's mic input is around -55dB (and that's on the 'power saver' mode I made to reduce interference). I'd imagine it's around the same on the output. Your built in soundcard may have decent input/output quality however.

Having said all this, I tend to just use my built-in soundcard and ASIO4ALL when doing stuff at home for convenience's sake. I only really use my external soundcard when I'm playing out.

Recording.

it is also quite good to have the proper output to drive your speakers with as well as a headphone port

aha great.
i have no ext.soundcard because i think my built in card is good.. i'm on a macbook, with standard soundcard, but now that you mention.. i do hear a noize when i record via my audio in.. i have a little bit of latency problems, but i think it's midi-latency. i just need to play a bit before the beat
i think i might consider buying an ext.one..
any recommendations?