It's useful to be able to recognise the weapons on a ship simply by looking at it - it lets you figure out roughly what someone's fit might be before you actually engage them, and can help you decide the best way to proceed in a fight. Recognising the difference between blasters and railguns, or medium and small guns on a cruiser, is likely to have a significant difference on the range you want to engage at or even whether you want to take the fight at all. Even different calibre guns within the same class can give you an idea of what other modules your opponent might be fitting (for example a rupture with 425mm autocannons will almost always be shield fit, while dual 180mm autocannons suggests a heavy armour tank).
To help with identifying weapons visually, I put together a chart of all the subcapital turret and launcher models. Have a browse through it, and then try looking at a few ships in space and see if you can figure out what they're fitting!
Update: Some people have asked about printing it to put on the wall. If you want to try it, here's a high resolution version which should look much better in print.