Organic Oil; aka Dormant Oil or Horticulture oil, is done usually in the winter to coat the tree in a thin oil sheen that suffocates overwintering insect eggs.
I feel it is a must because it IS organic, inexpensive and good to coat open wounds after winter pruning.
The truth about Oil is that, although not toxic like a chemical insecticide, it is more difficult to apply because of the amount of water volume needed to do it right. The common mix rate is 1 to 100 or in most cases one gallon to 100 gallons of water. It has to be applied to "run-off" which means to get adequate coverage you need to really use a lot to cover the entire tree correctly.
The photo shown is not enough material to really do the job, even a small fruit tree can use as much as 7 to 8 gallons to reach complete run-off and a large Maple could use as much as 30 gallons.
This means you either buy a dedicated thirty gallon tank with a gas pump that can shoot the material thirty plus feet high, or you hire a professional. The material cost is not that much so what you really pay for is the $5,000 of equipment to apply it right. Still, that fruit tree shouldn't be more than $10 a tree after the minimum stop fee (price to get the equipment there). I do this with a large pump system in a truck and still the average yard costs is $65.
The other important item about the equipment is agitation. This is oil and water...we know that old saying, they don't want to mix. So the tank must be aggressively agitated the entire time, so the guy in the photo should be shaking that tank continually.
My advise is to have the bigger equipment do the organic oil sprays so you know it's being done right. Often this is all you need in pest control for the tree. If a situation arises where you are forced to do an insecticide on your fruit trees, the above photo would be fine because you'll be using a lot less material and just misting the product on (not to run-off).
If you've pruned...make sure the oil is sprayed...insects love to lay eggs on new wounds.