I held off for quite a while before I got myself a yoga brick. It's not that the things are expensive; pretty much every yoga brick on the market is under $10. Rather, it's that you're paying money for a rectangle. You could stack up books, or use a shoebox, or get a rock. Why would you pay for a "fake" brick made out of foam?
The more I get into yoga, the more I realize that sometimes these little purchases do help out a lot - and if you space them out over several months, it's really not a big deal at all. A yoga brick is very helpful in many different poses. Yes, you could put your hands down on a pile of books when you stretch towards the ground from a standing position, but the books will slide and not support you well. Yes, you could sit back on a soft pillow when you go to stretch your arms behind you, but a brick gives you much more firm support and it means you can stretch a big more firmly with that resistance.
No, it's not necessary by any stretch of the imagination. A strap isn't necessary either. But I love my strap and use it every chance that I can. The brick really does help in a number of poses, letting me stretch just a little bit further, or stand just a little more tall. Every inch helps to improve my flexibility and strength.
Being a rectangle, each of the 3 sides of the brick is a differnet length. That means you can rotate it to best help you in what you are trying to do. For example, I put it lengthwise between my feet when I am sitting back, and it "raises me up" off the floor so that my stretches down can be even more long. It lets me start from a "higher position" sometimes with my hands on the brick, so that the stretch down my legs is even more helpful for me. It is great for beginners who can't touch their toes yet, because it means they have something to support their bodies on while doing various poses.
So again, not something critical to starting yoga, but a nice add-on once you get into the routine.
As a final comment, the photo of this item looks blue-ish but in reality it is a very nice royal purple color.