A way to bend pipe with a kick looks very good on radiators especially and saves using a fitting. Normally done in the second year of a plumbing course in the UK.
Because the TRV sits off the wall further out than the pipe in the clips. If you did a 90 bend from the clips it would likely sit behind the TRV which is why you would need to do a kick (also known as a dog leg) so that you can line it up with the TRV. Also as it might be easier to measure the inside edge of the pipe rather than the centre to get a more accurate reading, and then line the mark up with the inside edge of the offcut pipe
