Question: I am a junior developer I can't understand how canvas tag is beneficial for us?
I read lot of articles on that but I can't get the root benefit getting from the canvas tag.

Answer: 
Think of the difference between canvas and svg as the difference betwee Photoshop and Illustrator (or Gimp and Inkscape for you OSS folks). One deals with bitmaps and the other vector art.

With canvas, since you are drawing in bitmap, you can smudge, blur, burn, dodge your images easily. But since it's bitmap you can't easily draw a line and then decide to reposition the line. You need to delete the old line and then draw a new line.

With svg, since you are drawing vectors, you can easily move, scale, rotate, reposition, flip your drawings. But since it's vectors you can't easily blur the edges according to line thickness or seamlessly meld a red circle into a blue square. You need to simulate blurring by drawing intermediate polygons between objects.

Sometimes their use case overlaps. Like a lot of people use canvas to do simple line drawings and keep track of the objects as data structures in javascript. But really, they both serve different purposes. If you try to implement general purpose vector drawing in pure javascript on top of canvas I doubt you'd be faster than using svg which is most likely implemented in C.
The cool projects that came to mind for me are:

* Visualize gps data. GPS data is just an XML list of coordinates. You could easily build something in canvas to "connect the dots".
* An mobile app where the user can actual sign a document with her finger - canvas allows you to export out the rendered canvas drawing to PNG where it can be saved on the server.
* In a game where you have avatars, you can allow the user to actual draw on the avatar. Moustaches anyone?

Other stuff:

* In iOS / Android using lots of CSS3
effects like box-shadow can lead to
poor performance, especially when
animating. You can do a lot of these
graphics in a single canvas tag,
here's an example:
<http://everytimezone.com/>. This thing is flawless on an ipad.
* Cool background effects. For example try going to Paul Irish's
site and move your cursor around the
background: <http://paulirish.com/>
* In this HTML5 book sponsored by Google a lot of the effects are using
canvas:
<http://www.20thingsilearned.com/> -
particularly the "page flip"
animations.