Essentially, when Season 8 came around The Practice was dead and everyone knew it. The show had run it's course, the stars quit (or were fired, depending on who you ask), and the network didn't want it anymore. But it got one more season anyway.
David E. Kelley, not being an idiot, seized the opportunity to continue his career of making quirky Boston-based legal dramas by turning the entire 8th season of The Practice into one giant pilot for a then unnamed spinoff.
New stars were introduced to replace the ones who had been lost, stars who were always intended to be move onto the new show. Most notably, Alan Shore. Spader was recruited as the new star of The Practice with the intent that he star in the spinoff when it was picked up.
The supporting characters of The Practice who remained were given closure, but the season is essentially Boston Legal season 0 as much as it is The Practice season 8. Some of the characters were tweaked dramatically between the end of The Practice and the beginning of Boston Legal, and the tones are very different (that wasn't the original intent, but it just turned out that way), and you don't really need to watch season 8 of the The Practice to understand BL. But it doesn't hurt.
Also, all of David E. Kelley's Boston shows take place in the same universe. This includes Ally McBeal, The Practice, Boston Public, and Boston Legal. Minor characters occasionally cross over, and major characters occasionally appear as guests in other series. He also likely to reuse actors for differenct characters, and these two facts combined can make things a little confusing.