As mentioned, the onboard GPU offers a very useful backup if you are without a discrete graphics card (maybe you are testing your system to see if the graphics card is at fault or without a graphics card for a few weeks due to an RMA). In these situations the HD 3000 GPU on the i5 2500K is very useful and it provides approx. the graphics power of a low-end GPU like the HD 5450.
It has also been mentioned that many people buy these CPUs not for gaming, but primarily for CPU intensive computing tasks like audio/photo/image/video editing and creating, CAD, software development and many other tasks. These people often need a powerful, but good value CPU - so the LGA1155 i5 and i7 quad core chips are a good option and the HD 3000 graphics core is good enough for their needs.
Also, Intel introduced a new technology with Sandy Bridge which uses the onboard graphics cores to greatly increase the performance of video transcoding - called Intel Quick sync. With the arrival of the Z68 boards and Lucid's Virtu technology (which allows you to access the onboard GPU and a discrete graphics card at the same time) a user with a graphics card can also make use of this powerful feature (so long as the software they are using supports it).
Therefore, if you have the choice between an i5 CPU with a graphics core (i5 2500K) or without one (i5 2550K) then I would strongly suggest you go for the one which includes the graphics core - as it is very useful to have, doesn't tend to cost any more and doesn't offer any downsides when in use.
If you are looking for a good value motherboard for a LGA1155 CPU then I would give the
gigabyte d3 board a good look. it is great value and gets
good reviews - though the second main PCIE port only runs at x4 speed and it doesn't support SLI, though if you want to run dual cards properly (PCIE x8/x8, CF/SLI support) then you need to pay more for a ~£100 board like
this one.