Many analog-style plugins (like compressors or tape emulations) are designed to "expect" a certain input level. Calibrating your VU meter to -18 dBFS ensures you hit the "sweet spot" of these plugins without distorting them.
The Deluxe version includes extra channel tools like high-pass and low-pass filters. download vu meter plugin
Klanghelm's VUMT is a favorite for its highly accurate "ballistics"—the way the needle mimics the physical inertia of an analog meter. download vu meter plugin
Because VU meters don't "flicker" as much as digital meters, they are easier on the eyes during long sessions. Just got WAVES VU METER. When and where do I use it? download vu meter plugin