I get the image through wordpress built in post_thumbnail, but i see a lot of developers use timthumb for that and i don't know why! is it more flexible or something?
There are two reasons:
- post_thumbnail was introduced with WordPress 2.9, and many themes which used TimThumb because post_thumbnail wasn't yet available to them.
- TimThumb is more flexible than post_thumbnail — it gives you more options for how images are cropped, and allows for simple filters to be applied to the images.
However, TimThumb has also caused security issues, especially with sites which didn't keep their themes (and because of this their timthumb versions) up to date. Because of this, it often makes more sense to use post-thumbnail, especially if you don't need much beyond WordPress' built-in cropping for thumbnails.
Yes, it is more flexible. It allows you to resize or crop image in way you want ( check it's parameter listing: http://www.binarymoon.co.uk/2012/02/complete-timthumb-parameters-guide/ ). And have a look at project's homepage also: http://www.binarymoon.co.uk/projects/timthumb/
On the other side, if you do not need to use advanced image manipulation's, WordPress feature image (post_thumbnail) is OK. I, personally, have never uset timthumb in my themes and they all are working good. Especially when you can add_image_size