Contemporary media coverage suggests they wanted to buy out a competitor which became the dominant video site in a year. This New York Times article [1] from 2006 wrote:

"The acquisition of the privately held YouTube will enable Google to thrive in one area of the Internet where it has so far failed to gain footing. According to Hitwise, which monitors Web traffic, [YouTube] has the lion's share of online video traffic (...) a 46 percent share, MySpace has 23 percent and Google Video has 10 percent."

Meanwhile, this 2006 article from the Economist [2] stated:

"the deal is 'an aggressive, mature move for Google, one that shows that senior management is not too proud or stubborn to see that they can't build everything themselves,' says Henry Blodget, an analyst at Cherry Hill Research. Indeed, Google's Mr Schmidt freely conceded that YouTube is the 'clear winner', especially in creating social networks around its site."

[1] http://www.nytimes.com/2006/10/09/business/09cnd-deal.html

[2] http://www.economist.com/node/8031159

Google knew that Youtube will turn into the largest video search engine, so they bought it strategically. Otherwise someone else could have swooped in and acquired the largest video search site, which would have left Google vulnerable in many ways.

Summary: userbase/community was what drove the network effect, which made it impossible for Google to catch up, and that's why Google made the decision to purchase Youtube.

> was there some technical aspects to YouTube that Google couldn't or didn't want to build themselves?

Google had very much the power of building a YouTube-like community themselves, but buying out a competitor is a short-cut since it has the positive side-effect of removing that competition in our business. After buying YouTube, Google didn't have to waste time in building a community around a new product or "converting" the YouTube masses to theirs.

This short-cut way of acquisition is nowadays preferred by large companies rather than risk everything by doing innovation and "real" competition.

Google had a YouTube competitor at the time. It wasn't very successful.

Google did build similar functionality. Google Video was a thing for a while. They closed it down a while after acquiring YouTube.

If I remember google video was superior in video quality and looser limitations, it was a place where you could commonly find full documentaries while youtube still had a 11 minute video limit. It was actually google acquiring youtube that caused youtube to loosen their limitations and go for HD. I doubt they bought it for any technical reasons, google's infrastructure was already superior.