It distributes traffic between three of Glasgow's most important approaches, and also provides a key link in the north-south drive through Scotland's central belt. Its main purpose is to help people avoid Glasgow. Thirty years ago this was an act of mercy, but now - thanks to the resurgence of Scotland's biggest city - it seems a bit harsh, and the M73 probably doesn't get invited to as many parties as it used to.
The M73 was here before almost all the roads it now connects to: opening in 1972 along with the section of M74 east of there, it originally met the A8 (which passed through a roundabout underneath it) and finished on the A80. It was only in about 1980 that the M8 arrived, bringing a much larger interchange; the early 1990s saw the M74 extended west (though it still didn't really go anywhere for almost another twenty years), and it was 2011 before the M80 arrived to finish off the northern terminus. If nothing else, you have to admire the M73's patience.
Scotland's motorway numbers are very easy to use (especially in comparison to the bizarre English system for allocating them): a motorway takes its number from the A-road it relieves. So M8 follows A8, M77 replaces A77, and so on. The M73 is, unfortunately, the odd one out: it doesn't directly replace any A-road. So it takes its number from A73, which is also a north-south road lying east of Glasgow, but which is in all honesty unrelated to this road in every way.
For thirty years, the M73 had only three junctions, at the M74, M8 and A80. Now it has a new local access near Gartcosh, which sounds like it should be a mild exclamation, as in "oh, Gartcosh! We're on the M73 and we'll have to miss out on the rejuvenated and vibrant city of Glasgow".