When you do your own painting projects, it's considered understood that you are going to be providing the paint for your project -- it's your own project, after all, and who is going to provide the paint if not you?
When you hire a professional painting contractor to do the painting job, on the other hand, it's not entirely clear whether you are meant to provide the paint or if the painting contractor will be bringing their own.
Though of course, you can certainly make suggestions to your painting contractor as to what kind of paint you want to be used, the only way to make sure that the kind of paint you want to be used is to provide it yourself.
People have different needs and want when it comes to paints, and of course, it is perfectly reasonable that a person would insist that only a certain kind of paint is used -- and this is really the best way to guarantee it.
One of the downsides of providing paint for your professional painting contractor is that the cost of the paint will be invariably higher than if you allow the painting contractor to purchase the paint for you.
Much like how it costs a lot more to buy oats when you buy it pre-measured in small cardboard containers ready to be taken to the office than if you buy it at a warehouse bulk supply store or even just a large container at a supermarket, paint costs more for the average consumer than it does for painting contractors because the painting contractor can buy in larger quantities and has deals negotiated with paint sellers.
If the price is more important to you than precise selection, you might be better off allowing the painting contractor to get the paint for you.
When it is time to acquire the paint, you can either be on the timeline of the professional painting contractor if you allow them to get the paint for you or your own timeline if you provide the paint yourself -- you are in charge of your own schedule to some extent and you can decide when to take the time to acquire the paint.
Since that is the case, you'll be able to get the paint exactly when you want.
It might go without saying that professional painting contractors know more about paint than the average person.
This is because they generally have years of experience that inform them as to what paints are appropriate for what situation -- not all paints are good for every paint job, after all.
The bottom line is that you might be better off providing paint for your professional paint contractor, but it's entirely possible that you would be better off allowing the painting contractor to buy it for you.
It really depends on your priorities in getting the paint as well as your budget and your own perceived expertise in the subject.