Dating from Anglo-Saxon times, Watford was probably originally a crossing place traversing the marshy Coln Valley and it was this valley that gave birth to the industry for which Watford became famous: printing. At one time at was known as the printing capital of the world.
The printing companies to be found in the town included Waterlow and the Sun Engraving Company. Odhams arrived in 1938 and dominated the town until it closed in 1983. The Coln Valley had been a centre of paper making since the eighteenth century. John Dickinson made Croxley Script nearby.
I write about the development of printing and paper making in my book Charlotte Bronte's Devotee about William Smith Williams the Reader at Smith Elder who first recognised her genius.
Watford is very much on the cusp between London and Hertfordshire. It is well connected to London being both on the LMS London to Birmingham line and also is on the London Tube map. historically its presence on the Grand Junction canal gave it access to both raw materials and markets.
Scammell trucks, which did so much heavy lifting in the Second World War with the Pioneer, tank transporters and recovery vehicles, relocated to Watford from near Liverpool Street in London.
Furniture makers Hille moved to Watford in the fifties and now manufacture in Ebbw Vale. de Havilland manufactured at what became Leavesden aerodrome.
Smith + Nephew has its headquarters in Watford.
Further reading:
Dennis F. Edwards, Watford: A Pictorial History. (Chichester: Phillimore, 1992