The Jazz Singer (1927)

Call me "out of date" but I've finally seen "The Jazz Singer" (1927) with Al
Jolson for the first time -- today! [Yeah, I can't believe it's taken this
long, either!] I must say it is quite an outstanding, fantastic movie!
I've heard in the past that it was a silent; yet, I read recently that it
was a talkie. Come to find out, it is BOTH -- mostly silent, yet, part
talkie! I think that is kind of bizarre, but I liked it tremendously!

Why did they manage to make part of it a talkie, yet didn't film all of it
that way? That puzzles me. Was it a financial issue, or was it
intentionally designed that way?

--Larry S.