Ny danger slicing pickles hours before frying them

If you are looking to fry veggies then yes, it is necessary to pre-heat. If you put them in a cold pan with oil, you start 'sweating' the veggies instead of frying them.

For example: if you put a sliced onion into a pan with hot oil, it will cook and have a nice golden brown color as it caramelizes. Put that same sliced onion in a cold pan with cold oil and then add heat, and the onion will first turn translucent and lose moisture.

Both of these cooking methods have their uses, but they are different.
How do you know when the oil is ready to cook? It will start to shimmer a little before it reaches the smoking point. Put the veggies in when you see this shimmer, or at the very first wisp of smoke.

There's a few reasons for utilizing this method:

1. You'll end up with a juicier meatball, as it is cooked in liquid.
2. It'll be rounder and more plump because it was cooked in a liquid.
3. You'll be 100% sure that it was cooked thoroughly without being burned.

The reason for the pan browning is just a reverse sear - purely for color/crunchiness and perhaps some flavor from a browned meat and the butter.