Well for a long time, many DAWs either didn't have a sampler, or didn't have one even remotely as fully-featured or easy to use as Kontakt. For Cubase users, it was pretty much a necessity.
Now that Cubase has its own samplers (both as the Sampler Track and Groove Agent), it's not necessary. But it still is way more robust and has great effects, so I still use it as my main sampler. The workflow isn't a world-beater, but the more you want to modify your sound, the quicker it becomes relative to using outside plugins, modulations, etc.