Just a transparent lump to stop the hands getting knocked off? Well yes, but wait, there's more: The suitability of a watch for tactical use is not just about the glass of course, but it does give you a good clue as to what the watch was intended for and whether it's a suitable accessory for your camouflage or black wrist.
The description of a watch glass, eg. "Mineral glass" doesn't mean much to most people. It's actually quite straight forward. Watch glasses come in 3 main types - mineral crystal, sapphire crystal and plastic. Since plastic glasses are the preserve of Mickey Mouse watches we'll only consider the first two.
Sapphire Crystal is usually regarded as the better as it is extremely hard and therefore scratch resistant, however there is an important disadvantage when compared with the cheaper mineral crystal - it shatters more easily. Therefore for a military watch mineral crystal is the normal choice.
The Seiko Sportura range SKA509P1 shown is clearly a fashion rather than practical watch and has a Sapphire crystal, whereas Casio's GShocks use mineral crystals.
A look at the Traser range shows where the watch glass was a fundamental design choice. Most serious Trasers have mineral crystal glasses, whereas the more lifestyle oriented Traser Classic Chrono BD Pro uses a Sapphire crystal.
The Suunto X10, although in a price range normally associated with Sapphire crystal glass also uses mineral crystal, but with a twist. Or rather a curve. To give the the glass greater strength it is slightly domed.
So the next time you walk into a Bond Street jeweller and ask for a cheap black watch for 6 months in Afghanistan, you can add a knowledgeable "and make it mineral crystal my man!".