Most playfields have lots of reflective white areas that are lit by the General Illumination circuits.
Even though you often don't directly see these areas, it's important to repaint them bright white.
My light meter says that by repainting these sections, you are gaining 30-40% reflectance - that's a lot.
Not only are you getting a brighter reflection, you are getting a pure reflection, rather than one tinted beige or yellow.
The old paint has yellowed, the old Lacquer on top of it has yellowed, but at some point these areas were indeed a nice white.
These areas are mostly out of sight of the player, so you can do them without a bunch of leveling and patching. Because you can do them so quickly, there is no reason not to do them.
Sometimes these areas are just clear wood on one version of the playfield and screened white on another, latter production run. It's a judgement call if you want the extra light reflection (and color purity) by painting the clear wood white. Some purists frown upon it, many customers insist on it.
Don't get paint in the light sockets, or try to put balls of foam or tape in them. Just put some old bulbs in the sockets and paint around them.