Historically, Apple has acquired company for two reasons – the people and the ideas the people have executed. They are interested in one thing: intellectual property that can be used to add value to thier existing products and services.
Siri had some amazing voice recognition software that was (and still is – I still have the app on my iPhone) leaps and bounds above what Apple created in their Voice Control app. Siri created a smart "lifestyle" app – one that let users talk to their device and do things in a way that was personal, engaging and "just worked." Voice Control was a public beta – the folks from Siri will definitely make it better.
Apple bought music sharing/streaming service Lala last year for the same reason – it offered a "your music, accessible through the 'cloud'" solution that I have every confidence that Apple will leverage for a future (next?) version of iTunes.