So I've been reading a lot of post (as well as member signatures) and have a question in regards to the reason for purchase of a certain lens. Lets keep all F stops constant on these lens I am about to mention. For example.....why would one feel it necessary for a 17-40mm when their is already a 18-55 kit lens? Or....why would one feel it is necessary for a 28-135 if they already have a 55-250 lens and a 18-55? One more....why would a 10-22 be considered rather a 11-16 if there is the option for the 18-55. I think you see what I'm getting at. If the focal length over lap each other why get an additional lens? Is it simply a convenience factor (although you'll have a ton of lens to carry around)? Are the glass that much better? Will the glass give it a big notable difference. Again....VERY noobish question, I was waiting to see if I can find it through search or another noob to ask but a curious mind asks questions haha.