Why exactly is BP and temperature checked before and after performing hemodialysis?

Dialysis vitals. Blood pressure and temperature are checked because changes can be associated with infection. Blood pressure also can drop related to fluid removal. Especially in patients with a permcath temperature changes may be due to transiet blood stream infections.
Dialysis. In hemodialysis there is a chance for the patient to become overcooled by the dialysate leaching heat from their body, there is also the fear of pyrexic reaction related to the dialysis. The blood pressure is important because if its too low you can't dialyze and dialysis with pumping blood out of the body and through a machine lowers pressure in and of itself and after could be low.