It does.
What it shows it a reference line and a moving indicator. If the indicator indicates "Slow" (IAS falls below MCP speed) you should increase thrust to keep the indicator on the reference line. Vice versa when you're going to fast... indicator is above reference line, so retard throttles a bit to bring it back. It's nothing more than a comparator if you insist on being technical.
It's probably the simplest instrument in any flightdeck, apart from maybe the whiskey compass
This is it, F/S bar with the little yellow indicator slightly to the left of the ADI:
http://www.!!!!!!!!!!!!!!/open.file/559521/L/
In this case the IAS/Mach matches the one set on the MCP, so the bar is centered. Every dot above or below the reference line accounts for 10KIAS, if I recall correctly.