I want someone to explain to me what are Thunderbolt 2 is used for? I know its to connect docks with multiple ports but at 20Gbps , in what case scenario do I want a Thunderbolt 3 with 40 Gbps?
Well, one example is the new LG 5k Thunderbolt display - a 5k display actually takes about 30 Gbps of bandwidth (equivalent to 2 DisplayPort 1.2 cables) so Thunderbolt 3 should (in theory) leave you with enough bandwidth to daisychain an Ethernet adapter or a hard drive from the display, and still have a '1 cable docking' solution. USB-C can't cut this (until GPUs and displays start supporting DisplayPort 1.3) - you'll notice that the LG
4K display, which is USB-C not Thunderbolt, can only support old, slow USB2 speeds on its USB ports.
Another use for Thunderbolt 3 would be high performance external disc drives such as superfast SSDs or large RAID arrays for tasks such as high-res video editing - but don't bother for your everyday backup hard drive or similar, which is probably slower than your USB3 connection.
There are also pro products for video and audio digitisation that can make use of Thunderbolt bandwidth.
Also, Thunderbolt is an extension of the internal PCIe bus from which all the other interfaces are driven. So, an ethernet controller, extra USB port, hard drive controller etc. connected by Thunderbolt is a "first class" device largely equivalent to an internal device, without the speed overheads and possible incompatibilities of something driven via an 'intermediate' interface such as USB. USB ports on a Thunderbolt dock, for instance, can be "proper" extra USB ports with its own controller, rather than connected via a USB hub that 'shares' a single USB port between several devices (which can sometimes cause issues).
Also, Thunderbolt can optionally use optical cables that are far, far longer than USB cables and less susceptible to electrical interference (nice if you want your 5k display & keyboard in the recording studio and the computer tucked away in the equipment room). That said, if you need to ask the price of optical TB cables, you can't afford them.
Quite frankly, though, from a practical standpoint, thunderbolt is rarely used and has been eliminated from the new MacBook Pros in favour of USB-C.
No - that was true of the 12" MacBook, but the new MBPs have Thunderbolt 3. Its just that whereas Thunderbolt 1/2 used the same connectors as MiniDisplayPort (and still supported MiniDisplayPort devices), Thunderbolt 3 uses the same connectors as USB-C (and still supports the things that 'regular' USB-C connectors do).
Odds are that the next version of the 12" Macbook will have Thunderbolt 3, too, since the next generation of Intel CPUs have a Thunderbolt controller built in.
However - you're quite right that its only "power users" that will actually need Thunderbolt 3 and the addition of USB-C makes this even more so. If you're not a video/audio pro, a cheaper USB-C or USB 3.1 external drive will be more than sufficient. Even for multi-port docks, USB-C should do the job
unless you want to run a 4k/5k display from the dock and really, really want that single-wire docking (what? plug
two cables into my MacBook? Is this some kind of shampoo ad!?)