Whenever you put a needle in a patient, always aspirate first, before doing anything else.
Literally no bad things can happen if you aspirate a little through a needle. You might get some blood, pus or CSF, but as long as you don’t start draining them dry, it’s most likely that nothing will come of it.
Literally terrible things can happen if you start shoving fluids through a needle whose exact location you’re not quite sure about, especially when giving subcut local anaesthetic. Aspiration will tell you where the needle tip is, be that in a blood vessel, under the skin or the subarachnoid space. Make a habit of it, and you won’t see your patient falling apart when you accidentally fill their heart with lidocaine.