Arabic is the second-most widely used alphabet around the world (according to Enclopaedia Britannica). Only Latin surmounts to it, which we've already known. Thus if we get learned Arabic alphabet, then we're ready with a remarkable percentage of the world.
Although Arabic is a Semitic language, using Arabic alphabet not only Semitic languages can be written, but also among others Persian, Urdu, Pashto, Malay, Hausa, Swahili, Panjabi, Uyghur, Kazakh, Kyrgyz, Azerbaijani, and Kurdish. Moreover the Jews living in diaspora often wrote Hebrew, and the Ottoman empire used to write Turkish language with it. The list is far from being complete.
In fact, the Arabic script with minor additions is suitable to represent any language. For instance in Europe it was also used to write Portuguese, Spanish, Albanian, Bosnian, Polish (Tatar minority), and Romanian language.
It even fits to write English. For example if we would write "Light comes from the East", it could appear as:
لايْت كَامْز فْرَم ذْ إِيسْتOf course, there's no standard spelling of Arabic script for English language, hence a slightly different spelling could also be used. But probably anybody - who got used to read Arabic alphabet - could relatively easily and properly read it, depending on the language he/she speaks, even though he/she never has met English before.
There are 30 different writing systems derived from Arabic script, like Xiao’erjing (小兒經, Xiǎo'érjīng, شِيَوْ عَر دٍ) for writing Chinese, and Aljamiado for writing Romance languages (Spanish, Mozarabic, Ladino).