If you use just the RF coax interconnects (with presets on the TV tuned to VCR, DVD etc.) then you lose quality due to the video signal being modulated onto an RF carrier and then demodulated again in the TV. The little modulators included in VCRs etc. are nowhere near broadcast quality, for obvious reasons, and in many cases do not provide stereo audio on the carrier either, so for best quality you want direct audio/video connections.

The SCART leads provide direct baseband video and audio links between the various pieces of equipment, which gives better quality than relying on the RF connections. SCART was designed to allow composite video, separate chroma and luminance signals, or separate RGB video signals.

The problem is that not all SCART sockets on every piece of equipment support every configuration. You can also find two SCART sockets on the same piece of equipment which offer different options on each: Some provide only signal inputs, some provide only outputs, some provide both, for example.

There is also a switching line, intended to have a TV automatically switch over to the SCART input when a signal is present (e.g. selects VCR input every time the VCR is fired up).

This range of possibilities on SCART connections makes the system far from the simple "Just plug the two together" idea which is promoted to the non-technical user. With "old fashioned" separate input/output jacks, it's much easier to figure out what's going on.

Here are a couple of links to the SCART connection arrangement:

The Peritel SCART Connector

SCART Connectors