The M10 was the original southbound distributor for the M1, now downgraded to A-road status. The reason that there was ever a short motorway with such an important number goes right back to when the neighbouring bit of the M1 opened in 1959. There were no other connections (there were, for that matter, barely any other motorways) so all the M1's traffic had to return to the old road network.
There was no way that any road in the vicinity could cope with six lanes of high speed traffic being dumped onto it. So at each end, a spur was built heading off towards a popular destination, with the hope that lots of traffic could be removed from the road before it slammed into the terminal roundabout. In the north, the M45 carried traffic onto the A45 to Birmingham; in the south the M10 dropped traffic at the A414 and A405, the predecessors to the M25.
In later years the M10 was a bit redundant, forming a third of a St Albans bypass. Ideally it could have continued a bit further to connect to the M25, alleviating congestion at the M1/M25 interchange, but this extension was never forthcoming.
When the M1 was upgraded to four lanes between the M25 and Luton, the M10 was killed off. Collector-distributor roads were built alongside the M1, and the A414 was routed on what was the M10, up the parallel carriageways alongside the M1 and off to join the next section of A414 at junction 8. The M10 shared the honour of being the second British motorway with the M1 and M45, but was reduced to A-road status less than a year before its fiftieth birthday.