A modern arcade monitor doesn't look like a TV, it is more like a computer monitor. Older game boards had very basic video circuitry, which was constrained in resolution for two reasons. The first reason is that most early monitors were based on TV designs, and so they had the same frequencies. Second, memory was expensive and higher resolutions require much more RAM. The red, green, and blue wires are the signals for the red, green, and blue electron beams in the CRT monitor.
TV combined all of the colors and synced them into the one RCA video line, that would be about 25% of RGB power. There were some very clever designs around, which used the concept of "sprites" to work around the memory limit, but resolutions were still limited. The simplicity produces sharply-defined but low resolution graphics. For an emulator to reproduce this exactly, the beam of the monitor must have a one-to-one correlation with emulators graphics memory.
Most new arcade monitors use VGA now but, the one thing that really set them apart from your computer monitor and TV is that they are designed with a frame for installing them in an arcade cabinet. This is a good thing because it really is the best way to mount the monitor in the cabinet. If you try to put a computer monitor in an arcade game, there is no way to bolt the it in.