The restraint needed to place the IV catheter or the discomfort related to injecting the sedative?
The most evident example of this is encountered in fractious cats.2.
When the pet's level of pain is such that having to move him or her in a position that will allow the placement of the IV catheter will cause too much pain.
Occasionally, there may be an excitability phase which can last a few minutes before the full effect of the sedative has been achieved.
Once the drug has taken full effect, the pet will be sleeping, peaceful and for all intents and purposes, out of consciousness and out of pain.
In most cases, it is less stressful to simply have the IV catheter placed without any pre-medications.